STonX
STonX was one of the first Open Source Atari ST emulators - the first version has been written back in 1995! So it even runs in a decent speed on quite old PCs like a 486 computer. The downside is that this emulator is not very accurate - GEM applications run very well and fast, but games and demos often do not work.
STonX runs on many UNIX-compatible systems with the X11 window systems - like Linux, BSD and so forth. But there are also ports to DOS and Amiga OS for example.
Visit the STonX homepage at http://stonx.sourceforge.net/ or the Nino's ancient STonX page at http://www.complang.tuwien.ac.at/nino/stonx.html for more information and downloads.