The Atari 8-bit family is a series of 8-bit home computers introduced by Atari, Inc. in 1979 and manufactured until 1992. All are based on the MOS Technology 6502 CPU running at 1.79 MHz, roughly twice that of similar designs, and were the first home computers designed with custom co-processor chips. This architecture allowed the Atari designs to offer graphics and sound capabilities that were more advanced than contemporary machines like the Apple II or Commodore PET, and gaming on the platform was a major draw; Star Raiders is widely considered the platform's killer app. Machines with similar performance would not appear until the BBC Micro in late 1981 and the Commodore 64 in 1982.
The original Atari 400 and 800 models were released with a series of plug-n-play peripherals that used Atari's "SIO" serial bus system, an early analog of the modern USB. To meet stringent FCC requirements, the early machines were completely enclosed in a solid cast aluminum block, which made them physically robust but expensive to produce. Over the following decade several versions of the same basic design were released, including the XL and XE series of computers and matching peripherals. All of these used the same basic logical design, but were of much lighter construction and less expensive to build.
The Atari 8-bit computer line sold two million units during its major production run between late 1979 and mid-1985, putting its sales on par with machines like the Sinclair ZX Spectrum and TI-99/4A. Its primary competition in the worldwide market was, starting in 1982, the Commodore 64, which is by far the best selling computer of the 8-bit era. Atari also found a strong market in Eastern Europe and had something of a renaissance in the early 1990s as these countries joined a uniting Europe. Some estimates place sales during this period at another two million units.
As the author notes on the site: Atari800MacX now supports Synchronized Sound, a Graphical Debugger, VAPI and PRO images, Copy and Paste from the emulator, Configuration files, and a new Hard Disk emulation scheme. There have been other enhancments and bug fixes as well... You can check it out and download from Atari800MacX homepage.
Rainbow was one of the first emulators available for Macintosh. Despite this, it has fairly good compatibility with original software. The Mac OS X version adds an improved GUI and significantly improved sound to the original version. You can download it from Richard Bannister's site.