"Yoomp!"
There was a programming contest for the atari 800 earlier this year. This was the winning entry. (For 500 euros) Take a look at the video. If I had an Atari 800, this is one game I'd love to have for it. Check out here for info on the 4 authors of the game. You can't even really say "For a homebrew it's a masterpiece." Hands down it's right up there with, or beyond, professional games made for the system back in the day. :)