Below are the recommended emulators for each platform.
for PC (Windows)
Name: ARAnyM
Version: 0.9.1
Release Date: 9th September 2009
Author: ?
Runs on: Linux Cygwin, Mac OSX, SUSE
Emulates: All Atari 32-bit computers, including ST
Price: Freeware
Website: http://aranym.org/
Local Downloads:
ARAnyM v0.9.1 Cygwin
Name: Steem
Version: 1.1
Release Date: 2006
Runs on: Windows XP, 2000, NT, ME, 98, Linux
Author: Anthony and Russ Hayward
Price: Closed-source Freeware
Website: http://steem.atari.st/index.htm