Rainbow is a closed source Atari 5200 emulator and an Atari 8-bit computer systems emulator developed by Chris Lam. While development has now ceased and the developers website is now gone, Rainbow left behind a strong legacy. For starters Rainbow was the first Atari 5200 Emulator to be ported across to the Mac OS X operating system, a porting job that was done by Richard Bannister.
Rainbow managed to retain a fairly strong compatibility rate, managing to emulate the vast majority of the official material for the Atari 5200, it usually only really came undone when software and games made use of functions that were completely undocumented. it is not cycle accuracy based like a lot of more modern Atari 5200 emulators are now that emulate even the most obscure functionality in a chipset.
Rainbow also featured interesting debugging tools for anyone who is interested in the inner workings of an emulator and the Atari 5200, as it includes Trace tools for viewing internal chip states and tracing 6502 execution.
The Mac OS X version is running an outdated version of the emulator due to 2.1 being a significant rewrite of the whole entire emulator, it should still be able to achieve reasonable compatibility and speed.