UAE is a mostly complete software emulation of the hardware of the Commodore Amiga 500/1000/2000.
A Commodore Amiga, for those who don't know, is a 16/32 bit computer system based on the Motorola 680x0 CPU and a few specially designed custom chips that provide very good graphics and sound capabilities. Its first incarnation, the A1000, appeared in 1985, followed by the highly successful A500 and A2000 models.
UAE is written for Unixish systems; it is developed on a Linux machine but it should compile and run on any half-recent Unix-like operating system. It has also been ported to a wide variety of machines and operating systems, including DOS, Windows 95/NT, MacOS, RiscOS, BeOS and NextStep.