Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
Nestopia - A portable and cycle-accurate NES emulator written in C++
ProSystem - The ProSystem Emulator is an Atari 7800 emulator for the PC and Windows OS.
FCEUX - FCEUX is a Nintendo Entertainment System (NES), Famicom, and Famicom Disk System (FDS) emulator.
Z26 - Z26 is one of the best emulators for the Atari 2600.
puNES - PuNES is a high accuracy NES emulator.