I recently (as in the other day) got into programming for the Atari 2600. My first project, which is coming along nicely, is actually a remake of a game I posted on ModDB years ago. I thus went to add the Atari as one of the systems for which this game has been developed, only to notice that it isn't an option.
If you head over to www.AtariAge.com and check out the forums, you'll see that the indie/homebrew development scene is alive and well. With a lot of people interested in older systems for nostalgia or just because they're cool, I'd guess there's development for other systems as well, and I think IndieDB and ModDB would be a great place for these communities to congregate!