The MBTA or T (w:Massachusetts Bay Transportation Authority) runs the local public transportation system that includes the subways, commuter rail, buses and water taxis. The subway in Boston and Cambridge is called the "T". If you are trying to find a subway station, look for a big black letter T against a white background. Most Cambridge stops, including Harvard Square, are on the Red Line. Hours of operation are roughly 5:20 am (6 am on Sundays) to a little after midnight. (The last outbound trains leave Park Street in Boston at about 12:45 am.)
Harvard University is located on the subway's Red Line at Harvard station. Pound Hall, the main Wikimania venue, is located at 1563 Massachusetts Avenue, a few blocks north on Massachusetts Avenue from Harvard Square. Pound Hall is the large building on the right-hand (east) side of the street. 
MIT is located a modest walk from the Kendall/MIT stop on the Red line.
The Red Line goes to or through many major points in Cambridge and Boston. Subway fare is $1.25. Bus fare is 90 cents. Buses require exact change, though many people give the driver $1 and receive no change. Subway stations have booths where you can get change. It is easiest to have exact change available. If you are going to be traveling by subway or bus frequently, it might be smart to buy some rolls of coins from a bank.
The MBTA is introducing a new payment system called the Charlie ticket that can be used for multiple rides. Not all stations accept them at the present time, however. Therefore, it is advisable to buy your tokens or passes at each station instead of loading up. There is a 3 day visitors pass that gives unlimited travel for US$18. This saves money on trip #15 which you will only do if you play hookey from the conference. You don't want to be cast into OuterWikiDarkness do you?
A brief list of the most popular stations you will use and what they take:
South Station: Charlie Ticket ONLY (tokens can be exchanged at vending machines)
Harvard Station: Tokens and Monthy Passes ONLY
Kendall Station: Charlie Ticket ONLY (tokens can be exchanged at vending machines)
Some stations, like Government Center, have lots of stairs--elevators or escalators may be hard to find or out of order.
The subway is usually pretty safe. It is always a good idea to keep your baggage very close to you and an eye on your belongings. Remember to take all of your personal items with you when you leave the trains.
Getting to Harvard and MIT from Logan Airport
Due to problems with the Big Dig's I-90 tunnel, use of public transportation to and from the airport is strongly advised.
Method 1 - Recommended subway route
Look for special Silver Line bus stops at the curb outside the terminal, at the far end in the direction of traffic flow. Note that there are two stops at Terminal B and the Terminal C stop (next to the w:AirTran airline check-in desk) also serves nearby Terminal D. The Silver Line will then take you to South Station, where you can transfer (for free) to the Red Line.
To get to Harvard, take the Red Line Inbound towards Alewife (not toward Braintree or Ashmont)
Double check - once you get into the subway-train-platform, you will see maps on the walls adjacent to the trains --- make sure Harvard is a destination on the map. After six stops and get off at Harvard Square stop. To get to the law school, follow these directions
Once you arrive at Harvard station, take the exit towards Harvard Yard; climb up the stairs, and you should see a T-Mobile and The Body Shop (perchance popular chain stores farmiliar to you) to your left and a giant brick walled structure with ornate iron fencing to your right. You can continue down the streetside (hot and unshaded by big trees) or walk into these brick gates (a shaded sanctuary from the sun), where the first building you'd see is called Harvard, to your left. Following that will be a bunch of residence halls, and as you head north along the routed map shown, you will eventually come to a grassy intersection. Follow the red-brushed road on the line in the map shown. (See also the aerial view version below...)
OR, if you're going to MIT, take the Red Line Inbound towards "Alewife" as above but get off at the Kendall/MIT stop (two stops before Harvard).
Silver Line and subway fare is $1.25; the subway fares are flat fees, independent of distance traveled. You will need exact change to board the Silver Line at the Airport. The trip from and to the airport by subway may take 45 minutes to an hour, including the time spent waiting for the trains.
The Silver Line, a bus with its own private tunnels, was redirected due to repairs to the I-90 Ted Williams Tunnel. As of Sunday July 30, the MBTA reports that service to and from the airport is running on a 10 minute interval (however, if you arrive at 5:30 AM EST, you may have to wait until 6:20 or so before a Silver Line arrives due to shortage of driver staff --- traffic is usually sparse in the early morning, however, the transfer to the Red Line my also result in subway train delay, perhaps due to the same reason).
Method 2 - Less direct subway route
Should there be new difficulties with the Silver Line, the following alternate subway route works pretty well:
A less direct way to get to Cambridge from Logan Airport by subway is to take the inbound Blue Line from the Airport station to either the Orange Line (State Street) or Green Line (Government Center) and transfer, then take either color to a station to transfer to the Red Line (at Downtown Crossing via Orange Line, at Park Street via Green Line). Massport, the agency that runs the airport, offers complimentary shuttle bus service between airline terminals and the Airport station on the Blue Line daily from 4 am to 1 am. 
Method 3 - Taxicab
A taxi ride from Logan Airport to Harvard Square will take about a half hour and cost about $35. Agents for unlicensed taxis sometimes (illegally) try to snare travelers walking through the terminal ("Excuse me, do you need a taxi?") Ignore them and look for signs saying "TAXI" to find the official taxi stand. Be warned that a taxi ride from Harvard Square back to the airport may take an hour or more, depending on traffic, because a recent Big Dig tunnel collapse has caused major bottlenecks. Specifically: there will be a long 1-lane tunnel span, which might choke up during traffic hour. Public transportation, e.g., the Silver Line bus, has its own private tunnel adjacent to the auto traffic; see Method 1 above.
Traveling from South Station
w:South Station is Boston's main terminal for intercity buses and trains (all Amtrak service, except from Maine, as well as commuter rail from the south and west). It is also a stop on the Red Line, which is the main subway line that goes through Cambridge. To get to Harvard Square from South Station, follow signs to the subway. The walk within South Station from the trains and buses to the subway is about five minutes, so plan your luggage accordingly. There is an elevator to the subway in the corridor behind the Amtrak information booth. Fare is $1.25. Take a train Inbound toward Alewife and get off at Harvard station. Travel time is about twenty to thirty minutes.
Note that the MBTA has increased weekday and weekend commuter rail service between South Station and Providence, RI, effective July 24.
Traveling from North Station
Some commuter rail lines and all Amtrak trains from Maine only stop at North Station. You can board the Inbound Orange Line subway there and transfer to the Red Line at Downtown Crossing. From there, take an "inbound" Red Line train to Harvard Square station; it's a free transfer.
Traveling between Harvard and MIT
From Harvard: Take the T INBOUND (the lower level) to Kendall/MIT. It'll be easiest if you go out the exit closest to the chimes, which will take you to an area with benches and trees. Go around to the other side of the building you just came out of, and you will be on Main St.--1 Cambridge Center is just across the St. If you get out at the other exit, you'll still be on Main St., just a bit further down--look for the huge Cambridge Center banners, the MIT Coop, or the MIT Press to find Cambridge Center close by.
From MIT: Take the T OUTBOUND--attention: the Kendall Square station's two tracks are NOT connected. To go Outbound, you MUST go downstairs through the entrance labeled Outbound, which on the side of Cambridge Center--to Harvard Sq. Take the exit closest to you and follow below instructions to get to Harvard Law.
The Silver Line, South Station and Harvard Station are wheelchair accessible. It's best to check with the MBTA to make sure all elevators are in service. See w:MBTA accessibility for more information.
Driving in Cambridge
Public transportation is a much better option than coping with Cambridge's one-way streets and expensive parking.
If you want to park near the Law School, one option is to arrive early enough to find a space at a meter on Mass. Ave. Many side streets have signs that say parking is for residents only--without a current Cambridge resident sticker, you will be ticketed. Be aware that the nearby parking meters have two hours limits so your car must be moved every two hours. Parking enforcement is intensive in the Harvard Square area and cars are marked and then ticketed if they have not moved in two hours, even if more coins have been put in the meter. Meters take US quarters only. We strongly advise that you not count on metered parking. On Sunday, however, meters and resident parking rules are not in effect, so driving is more of an option.
Harvard Law School has driving directions available on its website: 
Perhaps the best option is to park at one of the MBTA park and ride garages and take the subway from there. These often fill up on weekday mornings but are generally available over the weekend and cost $5 per day. The one at Alewife station is particularly convenient. It is just three stops on the Red Line from Harvard station. From the Massachusetts Turnpike, take I-95 north to the Route 2 east exit and follow Route 2 to exit 61.
Taxis are readily available at the airport. Agents for unlicensed taxis sometimes (illegally) try to snare travelers walking through the terminal ("Excuse me, do you need a taxi?") Ignore them and follow signs saying "TAXI" to find the official taxi stand. A taxi ride from Logan Airport to Harvard Square will cost around $40, including the tip (usually a bit more than 10%).
UPDATE: The new "Big Dig" tunnel connecting the Mass Pike to Logan was closed indefinitely, after a fatal ceiling collapse on July 10, 2006. Getting to Cambridge by taxi is no problem, but getting back from Cambridge to the airport now takes longer (and costs more) than the reverse trip. So bear this in mind when planning your departure.
Your best option for finding a taxicab in Cambridge is to look for a taxi stand (near any subway station) or to telephone one of the local cab companies such as Ambassador Brattle (617-492-1100) or Checker (617-497-9000.) It is only occasionally possible to flag one down on the street.
To get to the main conference, you should ask the taxi drivers to take you to Harvard Square, at which point please follow the "Getting to Harvard Law from Harvard Sq." instructions!
To get to Hacking Days, ask for 1 Cambridge Center.
Making it to your Final Destination
Getting to Pound Hall (reg. desk for Thursday-Sunday) from Harvard Square
If you took the T to Harvard, there will be two ways to exit the station. The one closer (by about half a block) to Pound Hall has stairs and an escalator roughly in the middle of the platform you arrived on. These take you to a mezzanine level where there are stairs to both sides of Massachusetts Avenue, either at or across from a Finagle-a-Bagel shop.
The other exit requires you to walk to the end of the platform and follow a ramp to a large lobby. This is the only wheelchair-accessible exit. There are escalators, stairs and an elevator from this lobby to the street. On the street, this exit is next to the Out of Town News stand and an information kiosk. There is also an office one level up on the way out where you can buy a "T" pass if you wish.
From the Finagle-a-Bagel exit: When you get to the mezzanine level turn right and go up the stairs. You will be next to Finagle-a-Bagel, on the west side of Massachusetts Avenue. Facing Finagle-a-Bagel, turn right and walk in this direction (north) along the west side of Massachusetts Avenue for several blocks. You will see a church on your left and then a large park (Cambridge Common). About half-way along the park you will see a cross walk where you can safely cross to the other side of Massachusetts Avenue. Do so and continue north to Pound Hall, 1563 Massachusetts Ave. It is a very square-ish building and will be on your right, a short distance in from the street.
From the Out of Town News exit: Cross Massachusetts Avenue toward the Harvard Coop building and turn right. Walk in this direction (north) along the west side of Massachusetts Avenue. You will pass Finagle-a-Bagel on your left. Continue as per the directions above.
From the bus stop: If you took the #1 bus to Harvard Sq., you will be dropped off at the Holyoke Gate to Harvard Yard. Cross Massachusetts Avenue toward Finagle-a-Bagel. Continue as per the directions above.
If you'd prefer a longer route that takes you through the Harvard campus and has fewer street crossings, follow these directions:
From the subway's Out of Town News exit: Head north (towards the area that looks like Harvard instead of a commercial area) and enter Harvard Yard (the part of Harvard with the high fences) through any of the open gates. Keep walking north--you should pass the John Harvard statue across the yard on your right--until you exit the yard. At this point, you should see the Science Center in front of you (it's a modern, 70s spaceship-esque building with observatories on the roof). You should see a path that you can turn left onto, which curves and has a railing. Turn left here, and follow the path until it takes you to Pound Hall, 1563 Massachusetts Ave. It is a very square-ish building and will be on your right.
From the Finagle-a-Bagel exit: You should have gotten out of the T-station by taking a left and going upstairs (following the sign that says "Harvard Yard". If you didn't and you end up on the same side of the street as Finagle-a-Bagel, cross Massachusetts Ave. towards Harvard to get to the other side. You should be able to see a gate very closeby--this is Johnston Gate, main gate of Harvard, and you should walk through it and take a left as soon as you pass by the first cluster of dorm buildings. Keep walking north until you exit the yard. At this point, you should see the Science Center in front of you (it's a modern, 70s spaceship-esque building with observatories on the roof). You should see a path that you can turn left onto, which curves and has a railing. Turn left here, and follow the path until it takes you to Pound Hall, 1563 Massachusetts Ave. It is a very square-ish building and will be on your right.
From the bus stop: If you took the #1 bus to Harvard Sq., you will be dropped off at Holyoke Gate. Go through the gate into Harvard Yard, and weave your way to the North-west corner of the Yard--easily done by going around Widener (the first building you will face) to the left, walking north (further away from the gate), turning left through a gap between the dorms and then turning right to exit the yard through the north. At this point, you should see the Science Center in front of you (it's a modern, 70s spaceship-esque building with observatories on the roof). You should see a path that you can turn left onto, which curves and has a railing. Turn left here, and follow the path until it takes you to Pound Hall, 1563 Massachusetts Ave. It is a very square-ish building and will be on your right.
Getting to the Berkman Center (registration for Tuesday/Wednesday) from Harvard Square
From the subway's Out of Town News exit: Cross Massachusetts Avenue toward the Harvard Coop building and turn right. Walk in this direction (north) along the west side of Massachusetts Avenue. You will pass Finagle-a-Bagel on your left. Continue up Mass Ave. (going north--make sure you stay on Mass Ave, which curves ridiculously) for several blocks. You will see a church on your left and then a large park (Cambridge Common). About half-way along the park you will see a cross walk where you can safely cross to the other side of Massachusetts Avenue. Do so and continue north until you get to Everett St., where you take a right. The Berkman Center is located at 23 Everett St., which is a cream colored building with a big white porch. We're on the second floor!
From the Finagle-a-Bagel exit: Take a right out of the subway station ("To Church St.") and walk up Mass Ave. towards the park (Cambridge Common). About half-way along the park you will see a cross walk where you can safely cross to the other side of Massachusetts Avenue. Do so and continue north until you get to Everett St., where you take a right. The Berkman Center is located at 23 Everett St., which is a cream colored building with a big white porch. We're on the second floor!
From the bus stop: If you took the #1 bus to Harvard Sq., you will be dropped off at Holyoke Gate. Follow Mass Ave. north (with the Church on your left and Harvard on your right) until you hit Everett St., and take a right. The Berkman Center is located at 23 Everett St., which is a cream colored building with a big white porch. We're on the second floor!
Getting to Cambridge Center (Hacking Days) from Kendall Square
Cambridge Center is a part of Main St., and 1 Cambridge Center is on the side with the MIT Coop, the Marriott, and the Outbound entrance for the subway. It is literally right next to the subway stop.
Cambridge and Boston are very walkable cities. Once you've arrived at your destination, you might find it easier to walk around than to try to take buses, the subway, or cabs. The Charles River has convenient shoreline paths and bridges near both Harvard and MIT.
Just how safe is Cambridge? Well, this area has a very low murder rate and attacks of people on the street are not very common. They do, however, happen occasionally, especially late at night. When you're walking around, particularly late at night, be smart. Stick to well-lit areas. Be aware of people around you. Keep an eye on your possessions. Travel with at least one other person whenever possible.
Using the public transportation system or taxi cabs is very safe. Unlike some other cities, there isn't a rampant problem with pickpockets here. All the same, be smart, be aware of others around you, and make sure you have all of your personal belongings.