Alex Mauer's "9999" features 42 songs using 9 different game system's soundchips. The list includes: NES, Commodore 64, Gameboy, Atari XL computer, MSX computer, IBM PC/DOS, Commodore Amiga, Game Gear, and Sega Genesis.
Wow. I'm really looking forward to the Sega stuff on there and I'm gonna get me one of these straight away. But if you still need some more convincing, you can hear a bunch of sample mp3s here.