Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
Altirra - Altirra is designed with emulation quality in mind, sometimes over speed and polish.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
Hatari - Hatari is an Atari ST emulator for Linux and other systems that are supported by the SDL library.
Hataroid - Atari ST emulator for Android based on Hatari.
Atari800MacX - This is the the Macintosh OSX Port of David Firth's Fantastic Atari 800 Emulator.