Hello, long time fan of this site (and store customer)!
I've been interested in the Atari homebrew scene for a long time, and a few years ago I came up with a project suitable for the Atari 5200. I chose the 5200 because it always had a legendary status in my mind from when I was a kid. I never had one myself, but knew a few people that did and it never ceased to blow me away.
I finally bought my own 5200 a few years ago and began collecting games. As a long-time game programmer, I wanted to try my hand at 5200 development.
Attending one of David Crane's talks at the Portland Retro Gaming Expo inspired something I've called Ratcatcher. It's a 1-3 player game featuring sewer workers running around a dangerous sewer collecting rats while avoiding alligators and other hazards.
The game was simultaneously developed for the 5200 and a number of PC platforms. I've licensed the project under the GPL3, so all of the source code is included. This is my first 6502 project, so the code is probably awful, but I still hope it might help someone out.
It's currently in beta, I expect to make a few gameplay tweaks. There are no known bugs, but I can't rule those out, of course.
I have a project page on my website with download links and screenshots for both the PC and Atari versions. All downloads include the full source tree for all versions.
If anyone wants to bang on the 5200 version, feel free. I'm not really accepting gameplay suggestions or anything like that, the game is what it is and it served its purpose (teaching me Atari 5200 programming). It really isn't very good, but I have some much better projects in mind for the future.