I have owned various Atari machines: 400, 800XL, STFM, 1MB STe. All are still in working condition, although they haven't been powered up lately.
STonX Homepage Atari ST Emulator running atop X Windows
The GEM graphics environment as well as much of the other Atari ST software produced by DRI has been released under the GPL by Caldera.
Llamatron by Jeff Minter was a hilarious fun videogame...
Run Atari ST software on a Palm OS5 PDA.