Stella - Stella is a multi-platform Atari 2600 VCS emulator released under the GNU General Public License...
Wine - Open Source Software for running Windows applications on other operating systems.
jGameBase - jGameBase is a fully-featured retro-gaming emulator frontend and game database utility.
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.
Hataroid - Atari ST emulator for Android based on Hatari.
WineBottler - WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps.