http://www.emulator101.com/introduction-to-chip-8.html
An average programmer can probably get an emulator for that running in a weekend, and there are lots of guides and documetnation out there for the opcodes and similar.
NES / Gameboy / GameGear / similar are probably well-documented, but they will be harder at least because the processors have more opcodes you have to care about.
Also somehow for some reason i was convinced back then that mobile phones in the future will have a CHIP8 emulator to play games :-P. Sadly(?) they got Java instead.
You are bound to have an awesome game you can find on an 8-bit system and it's really fun trying to get to the point where you can play the game: getting the logo to appear, getting the title screen up, getting the sprites working, adding controls etc.