Government sites
FCC (Federal Communications Commission) -- Keep tabs on what the folks in Washington are up to.

Ham Radio Organizations:
ARRL (American Radio Relay League) -- The national association for hams in the US.
RAC (Radio Amateurs of Canada) -- Canada's national ham radio organization.
AMSAT (Radio Amateur Satellite Corp.)-- This is the place to learn about ham radio satellites and keep up to date on the latest developments.
K4ABT's 'SEDAN' Page CQ Magazine Packet Editor Buck Rogers, K4ABT, has an excellent page on all aspects of packet radio, with a special emphasis on SEDAN -- the South Eastern Digital Association Networks (after all, it is the SEDAN home page!).
TAPR (Tucson Amateur Packet Radio)-- TAPR pioneered packet radio in the United States, and this is a great place to go for basic or advanced packet radio info.
The VE7TCP Web Page -- This is a great reference for just about anything ham-related. Sections include Contesting, DXing, Packet Radio, Propagation, Technology and much more!
10-X (Ten-Ten International Net)-- Ten-Ten is devoted to promoting activity on the 10-meter band. Members receive and exchange unique membership numbers to qualify for awards and for contest points. If you enjoy 10 meters, you should know about 10-X.