Atari 8Bit and Mac - Atari800MacX
http://www.atarimac.com/atari800macx.php
"This is the home page of the Macintosh OS X Port of David Firth's Fantastic Atari 800 Emulator. I have ported the SDL version of the Atrari800 emulator to Mac OS X, and added a full native Cocoa interface, including Preferences, Menus, File Associations, Help and more."
YouTube - Atari 800 in store demo
http://youtube.com/watch?v=jgT8279DVGQ
"one of the first Atari demos to show 128 colors on the screen (the beginning logo) and that was actually good music for a computer back then."