Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.
Z26 - Z26 is one of the best emulators for the Atari 2600.
Hatari - Hatari is an Atari ST emulator for Linux and other systems that are supported by the SDL library.
WinUAE - WinUAE is a Windows port of the emulator UAE.