Atari800MacX is described as 'This is the the Macintosh OSX Port of David Firth's Fantastic Atari 800 Emulator. Ported is the SDL version of the Atrari800 emulator to Mac OSX, and added a full native Cocoa interface, including Preferences, Menus, File Associations, Help and more' and is an app in the Gaming Software category. There are more than 10 alternatives to Atari800MacX for a variety of platforms, including Windows, Linux, Mac, BSD and Self-Hosted solutions. The best alternative is WinUAE, which is both free and Open Source. Other great apps like Atari800MacX are VICE (Free, Open Source), Stella (Free, Open Source), Z26 (Free, Open Source) and MESS (Free Personal).
jGameBase is a fully-featured retro-gaming emulator frontend and game database utility. It is a sister application to the Windows program GameBase (without "j"), the official frontend for the GB64 Collection.
Power20 is an emulator for the Commodore VIC-20. It allows you to run your favorite VIC-20 software on your Apple Macintosh (with PowerPC or intel-CPU) at full speed with smooth graphics and great sound.