Wine - Open Source Software for running Windows applications on other operating systems.
Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
CodeWeavers CrossOver - Don't buy a Windows license, don't reboot and don't use a Virtual Machine. Try a free trial of CrossOver to run your Windows software on Mac, Linux and Chrome OS.
Z26 - Z26 is one of the best emulators for the Atari 2600.
WineBottler - WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps.
Javatari - Javatari is a multiplayer Atari 2600 emulator written in pure Java with no external libs.