I'm not the greatest Atari 2600 programmer, but I made some hacks about 10 years ago of Pac-Man (actually Ms. Pac-Man, but I made a better Pac-Man out of it, modifying the board layout, sounds, graphics, colors, and changing the gameplay appropriately) and Space Invaders, both of which were made into cartridge form for a while. They've both been hugely outshone by homebrew versions of the games that others wrote from scratch, like Space Instigators and Hack'Em, but at the time I was very proud of them.
Originally Posted by the2ndflood
I also wrote some demos from scratch including Boing 2600 (as in the old Amiga bouncing ball demo) and a couple different horizontal scrolling demos which I meant to develop into a port of Ballblazer. I gave up at the time for various reasons, unrelated to the difficulty of the coding, but someone else has picked it up recently and is doing amazing work which he posts on AtariAge.
Don't think I can include links until I make a few more posts, but if you google for raindog old videogames my page will be pretty close to the top. (And the personal ad immediately underneath it is not me.)
Anyway, I've been looking for a truly portable Atari 2600 solution for a long time, and this is the best yet. Thanks again to debilator for his work on it.