Frodo - Frodo is a robust Commodore 64 Emulator for the Macintosh.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
VICE - Bring loops to life with the loop slicer plugin for Mac and Windows.
Z26 - Z26 is one of the best emulators for the Atari 2600.
C64 Forever - C64 Forever is a revolutionary Commodore 64 emulator, preservation and support package.
Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.