Pilots Guide to Boston TRACON - Everything you wanted to know about Boston Approach, but were afraid to ask.
GTE DUATS - Free of charge flight planning tool for certificated pilots. You can even download Cirrus, a windows based interface to DUATS.
National Weather Service - Great site for weather forecasts - you can even get current conditions at several airports (METAR), national satelite an radar images.
AirNav - Provides free detailed aeronautical information on airports and navigational aids in the USA. (Suggested by Eric Poole - thanks!).
Aircraft Owners and Pilots Association - A great resource for members as well as non-members and student pilots.
Landings - A collection of news, weather and aviation related sales - sort of like an "aviation portal"
Atlantic Flyer - The web-version of the magazine you see at Eagle and other FBOs.
Avemco - Offers renter's insurance. Required by Eagle East Aviation.
AeroPlanner.com - A comprehensive flight planning tool on the web. Unique "TripTick" PDF downloads for each leg of your flight.