RetroArch is a frontend for emulators, game engines and media players.
Versatile Commodore Emulator
An advanced network simulator to design and configure virtual networks
Run Windows applications on Linux, BSD, Solaris, and Mac OS X
xBRZ: "Scale by rules" - high quality image upscaling filter by Zenju
C64/Amiga emulator with shader and runAhead
C64 Emulator
Hatari is an emulator for the Atari ST, STE, TT and Falcon computers
Emulator of the 8-bit computers Sharp MZ-800 / MZ-700
NES/Famicom emulator based on FCE Ultra.
A portable library and tools for archiving images of Atari 8-bit tapes
Create your own Atari tapes
Z80 shared library including some nice test-projects