Atari800MacX is an Atari 800, 800XL, 130XE and 5200 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. It does not require libSDL to be installed separately, but includes it within the application package.