Publisher: Activision

Activision was the first company other than Atari to sell games for the Atari 2600. It was founded in 1979 by programmers who had left Atari, among them David Crane, Alan Miller and Bob Whitehead, and it started out in Crane’s garage. Its first four cartridges, Boxing, Dragster, Fishing Derby and Checkers, appeared in the summer of 1980. Many more followed from designers such as Carol Shaw, Steve Cartwright, Garry Kitchen and Larry Miller. Activision kept making 2600 cartridges long after the 1983 crash, including late arcade conversions such as Kung-Fu Master, Commando and Double Dragon.
The site has 50 pages from Activision, including several unreleased prototypes. A few to start with:

River Raid (1982): Carol Shaw’s river shooter, the second best-selling 2600 game of 1983.
Pitfall II: Lost Caverns (1984): David Crane’s underground sequel, with swimming, balloon rides and a full musical score.
Seaquest (1983): rescue divers before your submarine’s oxygen runs out.
Kaboom! (1981): catch the Mad Bomber’s bombs in water buckets, using the paddle controller.
H.E.R.O. (1984): fly a backpack helicopter down mine shafts to reach trapped miners.
Enduro (1983): a long-distance race through day, night, snow and fog.

Browse the full list below and play any of them in your browser.