Three previously unreleased Atari 2600 prototypes have been discovered and dumped online by a poster over on the AtariAge forums (as spotted by John Hancock!).
The AtariAge poster, who calls himself Ianoid, posted in the AtariAge forums earlier this month, publishing links to download six different cancelled Atari 2600 titles that the developer UA Ltd had at one time planned to release for the system in the early 80s (all of which were based on their previous offerings for the short-lived Emerson Radio console, the Arcadia 2001). It's speculated that they were cancelled due to the video game crash of 1983 and being unable to compete in a heavily-saturated market.
Three of these (Cat Trax, Funky Fish, and Pleiades) were previously made available to buy for the Atari 2600 from the AtariAge store back in 2003. However, the remaining three have never been released to our knowledge.
These include a port of Sigma Enterprises's 1980 coin-op R2D Tank, the Frogger-esque original title Hobo, and a take on another Sigma Enterprise arcade game called Spiders. As shown in John Hancock's video on the titles, these games aren't as advanced as their Arcadia 2001 counterparts but still look to be enjoyable arcade experiences worth checking out if you happen to own an Atari 2600 and have been looking for something new to play.
You can download all of the games from the AtariAge forums. More information on these games is also available on the website AtariProtos.