CrabEmu is a Sega Master System, Sega Game Gear, SG-1000, ColecoVision, and NES emulator for Mac OS X (10.3 and newer), Linux/BSD/etc., and Sega Dreamcast.

 

Note that CrabEmu focuses mainly on the first few consoles up there (the Sega ones). ColecoVision support seems to work pretty well, but NES support is nowhere near complete.