RetroArch - RetroArch is a frontend for emulators, game engines and media players.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
Mesen - Mesen is a NES/Famicom emulator and NSF player.
Z26 - Z26 is one of the best emulators for the Atari 2600.
Snes9x - Snes9x is a portable, Super Nintendo Entertainment System (SNES) emulator.
Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.