Atari Guide - The Best Info site for 2600 carts!!!
Atari Age - One of the most complete Atari sites I've seen!
Atari 2600 Landfill - Games for trade, Rare Cart Guide, Plus lots more!
Atari Gaming Headquarters
Atari Central - This one has Atari Message Boards!
Atari Historical Society - Atari Museum
Games - Sale of vintage systems and games for Atari plus many other systems