Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
Mesen - Mesen is a NES/Famicom emulator and NSF player.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
Nestopia - A portable and cycle-accurate NES emulator written in C++
Steem Engine - Steem is an open source Atari STE emulator for Windows and Linux.
VirtuaNES - VirtuaNES is Famicom/NES emulator for Win32.