Inside the Homebrew Atari 2600 Scene. Links from the article: Stella, an Atari 2600 emulator available for many platforms. A list of homebrew games. A beginner's guide to Atari 2600 programming.

Seems like this would only be truly satisfying if you had access to an original machine an an EPROM cartridge. I'd be up for Commodore 64 homebrew if I still had one; programming just for an emulator seems pointless. Constrained programming for a small modern platform, such as the Gameboy Advance, might be just as fun.