Linux. List of working Emulators:

System -> Emulator
Amiga -> e-uae
Amstrad CPC -> sdlmess
Apple II -> sdlmess
Arcade Games -> sdlmame
Atari 2600 -> stella
Atari 7800 -> sdlmess
Atari 800 -> atari800 emulator
Atari Lynx -> mednafen
Atari ST -> hatari
Coleco/Adam -> sdlmess
Commodore VIC20 -> VICE, sdlmess
Commodore PET -> sdlmess
Commodore Plus4 -> sdlmess
Commodore 64 -> VICE, sdlmess (required a source code patch for joystick)
Commodore 128 -> VICE, sdlmess (required a source code patch for joystick)
Fairchild Channel F -> sdlmess
Gameboy -> mednafen, gnuboy-sdl
Gameboy Color -> mednafen, gnuboy-sdl
Gameboy Advanced -> mednafen (required a source code patch)
MSX Computer -> openmsx
NES -> mednafen, fceultra / gfceu
NeoGeo Pocket Color -> mednafen
Odyssey 2 -> sdlmess (not 100% accurate)
Sega Genesis -> sdlmess
Sega Master System -> mednafen, sdlmess (smsj)
Super Nintendo -> snes9x / snes9express
TRS-80 -> sdlmess
TurboGrafx-16 /PC Engine -> mednafen
PC MS-DOS -> DOSbox, QEMU
PC Windows 3.1/95/98/XP -> QEMU
Vectrex -> sdlmess
ZX Spectrum -> sdlmess

That leaves a few I haven't been able to run successfully:

Atari Jaguar (almost runs in sdlmess -- not fast enough or 100% accurate)
Intellivision (should be working in sdlmess, I think, not sure why not)
Nintendo 64 (if possible would probably require accelerated video)
Nintendo Gamecube (if possible would require accelerated video)
Nintendo DS (desmume might work eventually?)
Sega Saturn (yabause runs very slowly, not playable)

http://ubuntuforums.org/showthread.php?t=316047

Cuz Ubuntu is the shit.