The Fuzebox is a fully open-source, DIY 8-bit game console. It is designed specifically for people who know a little bit of programming to expand into designing and creating their own video games and demos and MUSIC!
The basic Fuzebox kit (just the circuit board and parts) will run you $70 (USD), while a complete starter kit with a controller, power adapter and a custom enclosure for $100. You can get your hands on the kit along with schematics, programming instructions and some sample code over at Adafruit