RetroArch is a frontend for emulators, game engines and media players.
PS2 Covers Collection
A JavaScript NES emulator
C64 Emulator
Run Windows applications on Linux, BSD, Solaris, and Mac OS X
A portable library and tools for archiving images of Atari 8-bit tapes
Hatari is an emulator for the Atari ST, STE, TT and Falcon computers
Z80 shared library including some nice test-projects
Game Boy / Game Boy Color Emulator Library
DOSBox fork for embedded devices
A multithreaded, test-driven electronics emulator for .net 4.0.
Complete, Fast, Accurate, Game Boy/CGB/SGB Emulator.
Modular, agent-oriented and extendable agent toolkit.
Upload games to legacy apple // computers via serial and no floppies!