Hatari is an Atari ST/STE/TT/Falcon emulator for GNU/Linux, BSD, Mac OS X, Windows and other systems which are supported by the SDL library. The Atari ST was a 16/32 bit computer system which was first released by Atari in 1985. Using the Motorola 68000 CPU, it was a very popular computer having quite a lot of CPU power at that time. Unlike many other Atari ST emulators which try to give you a good environment for running GEM applications, Hatari tries to emulate the hardware of a ST as close as possible so that it is able to run most of the old ST games and demos.
sz81 is a Sinclair ZX80/ZX81 emulator very much based upon the work of Ian Collier's xz80 and Russell Marks's z81 but employing the highly portable SDL and including additional functionality and features for desktop computers and portable devices
ScummVM is a program which allows you to run certain classic graphical point-and-click adventure games, provided you already have their data files. The clever part about this: ScummVM just replaces the executables shipped with the games, allowing you to play them on systems for which they were never designed!
The Atari 2600 Video Computer System (VCS), introduced in 1977, was the most popular home video game system of the early 1980's. Now you can enjoy all of your favorite Atari 2600 games on your PC thanks to Stella!
Fuse (the Free Unix Spectrum Emulator) was originally, and somewhat unsurprisingly, a ZX Spectrum emulator for Unix. However, it has now also been ported to Mac OS X, which may or may not count as a Unix variant depending on your advocacy position. It has also been ported to Windows, the Wii, AmigaOS and MorphOS, which are definitely not Unix variants.