I've used Easy 6502 a ton. A few years ago, I got into Atari 2600 programming. I personally haven't made anything useful but the community is very active and doing truly amazing stuff. I use desktop software (Stella and DASM) these days but 8bitworkshop[0] is a great site for playing with it.

[0]https://8bitworkshop.com/v3.9.0/?platform=vcs&file=examples%...