Acorn Atom emulator for MacOS Classic (PowerPC). Source code available.
A "Circus Atari" clone; despite the name, it is available for numerous platforms and hot on the heels of xrick for the title of "most ported game ever".
MSX emulator for Mac OS.
Emulation of Manchester University's Baby for Mac OS Classic.
Port of Amiga emulator UAE to Mac OS Classic.
Thomson MO5 emulator for Mac OS.
SNK Neo Geo Pocket & Neo Geo Pocket Color emulator available on many platforms.
Mac OS port of Neo Geo Pocket Color emulator Neopocott.
Neopocott is a relatively new NGPC emulator, with limited compatibility at present and no support for sound. However, it can emulate Sonic the Hedgehog properly, and that's all that matters!
Atari ST emulators NoSTalgia (for Mac OS X) and PowerST (for Mac OS Classic).
SSEM-Mac is an application emulating the Small-Scale Experimental Machine. Also known as the Baby, it was the first computer to run programs stored entirely in memory. SSEM-Mac requires any PowerMac to work, Appearance and 2 mb of RAM.
Mac OS port of Thomson TO7 emulator Thom.