Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
Steem Engine - Steem is an open source Atari STE emulator for Windows and Linux.
Z26 - Z26 is one of the best emulators for the Atari 2600.
Altirra - Altirra is designed with emulation quality in mind, sometimes over speed and polish.
Hataroid - Atari ST emulator for Android based on Hatari.
WinUAE - WinUAE is a Windows port of the emulator UAE.