Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
Mame - MAME stands for Multiple Arcade Machine Emulator.
WinUAE - WinUAE is a Windows port of the emulator UAE.
Z26 - Z26 is one of the best emulators for the Atari 2600.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
Hataroid - Atari ST emulator for Android based on Hatari.