Patty asked how to get from Logan Airpot to Cambridge. This one, luckily, is super-easy :)
If you're feeling rich, or have a lot of bags, take a cab. Tell the cabbie to take you to Cambridge -- if he/she asks for a route, tell 'im to take 90 to get there. Depending where in Cambridge, telling them "Central Square" may work well -- a lot of the tech conferences are at the Hotel@MIT, which is just down the street from Central Square (down Mass Ave). Also, coincidentally, it's literally across the street from where I work :) A cab ride to Cambridge will cost about $30 from the Airport, with tip.
If you're feeling less rich, and don't have a ton of luggage, then you're going to want to take the "Silver Line". The Silver Line is a bus that runs from the airport to various subway stops: OpenStreetMap shows it relatively nicely. You're going to take that to South Station: from there, you're looking to get on the Red Line heading towards Alewife. You do not want to be going towards Braintree or Ashmont. The Silver Line will dump you at Surface level -- you're looking for the Subway tunnel out in front of the South Station Train Station.
Once you're on the Red Line, you'll likely be looking for Kendall/MIT, Central Square, or Harvard, depending where your target is.
The Silver Line to Red Line route is really easy, and almost everyone I know takes it. (Jess usually insists on picking me up, so I don't take it that often. :)) It shouldn't be too difficult to follow, and is The Way to get to Cambridge from the Airport.