Quadrun: Atari 7800 development info and tools.
Mitchell Orman's Atari 7800 Page: Lots of good stuff. Manuals, cart lists, screenshots gallery, etc.
Atari 7800 Programming mailing list: Mailing list for people developing homebrew games for the Atari 7800.
7800 diagnostic cart analysis: Analysis and partial disassembly of the Atari 7800 diagnostic cart, along with a pin diagram for the diagnostic dongle and a list of the error codes and their meanings.