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.
Z26 - Z26 is one of the best emulators for the Atari 2600.
Steem Engine - Steem is an open source Atari STE emulator for Windows and Linux.
Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.
Hatari - Hatari is an Atari ST emulator for Linux and other systems that are supported by the SDL library.