Emulator

An emulator is a software program that tries to emulate / simulate another computer system than the one its running on.


 * Wine: some people will argue about the definition of emulator, and say that wine is not really an emulator if you are very strict it's just "an Open Source implementation of the Windows API on top of X and Unix."


 * Squeak: The Smalltalk-80 system does not have native hardware to run on... Well the closest native hardware documented is the Swamp processor, it is unknown is anyone have manufactured one. The inventor team of Smalltalk at Xerox PARC is currently working on http://www.opencroquet.org that enable object syncronisation across the internet with OpenGL 3D API and more. Interesting stuff all based on Smalltalk.

Please extend the list.
 * Xebian: Also not really an emulator, but i think it fits here because the host system (XBox) was never intended to run Linux or any other free OS and the community spend quite some time and effort until they got it running.