atarimania.com
Atarimania is a non-commercial database covering Atari 2600, ST, 400/800/XL/XE, 5200, 7800, Lynx and Jaguar software, with screenshots, box scans, manuals, adverts and magazines. The site states it hosts a detailed listing of all 2600 cartridges and a database of all Atari 8-bit software.