Kat5200 is an open source Atari 5200 Emulator and also an Atari 8-bit computer emulator (800/XL/XE). The emulator relies on the Atari 5200 BIOS files to be able to actually successfully run games as it does not emulate the BIOS itself within the emulator.
Kat5200 is regarded as an Atari 5200 emulator with advanced features, NTSC/PAL support, a GUI and good compatibility. It proved to be a very promising emulator up until it ceased development in 2009.
The last few versions of Kat5200 before it ceased in development appeared to focus on trying to increase the level of compatibility of the emulator, with version 0.6.0 adding support for the Atari 8-bit computers and adding support for PAL.