Boston

There are at least four ways to get to Boston for a given weekend.

1. During breaks in the academic year (Thanksgiving, Winter Break, Dead Week, Spring Break, and Summer Break), the Williams Transport Motor Coach service runs to Boston. Tickets cost $26 each way. You can reserve them online at http://motorcoach.williams.edu starting about 12 days before each break. The coaches used are comfortable, leave on time, and are less expensive than other options that leave from Williams. However, if you're not traveling during one of the 5 major breaks you'll have to use one of the methods below.
2. The Peter Pan/Bonanza bus leaves out of the Williams Inn at 10 AM every morning and 4 PM every afternoon. There are stops in Pittsfield and Springfield. Each way takes four hours and costs $44. There is no round-trip discount. You can reserve a ticket by calling up the Williams Inn, or just showing up to the front desk a few minutes before the bus leaves. It is not the most pleasant ride, however. On the way back, a student was left at the Pittsfield station late at night and told by the bus driver that the next bus may or may not show up that night (taxi costs $40 from Pittsfield to Williamstown). Also, prepare to be pan-handled in Springfield and/or stabbed, as this happened to a man a few hours prior to my arrival at said bus station.
3. Amtrak runs a line from Pittsfield (leaves 2:06 PM) to Boston (arrives 6:20 PM) that costs $27 round-trip. It tends to be up to several hours late, but is a comfortable ride for the price, should the planets align such that the train shows up before you've given up and left. However, you'll still need to figure out how to get to Pittsfield.
4. Find someone with a car and offer to pay for gas. A good way to do this by posting on WSO's Rideboard

See also How to switch from North Station to South Station.