ST
Atari ST Emulator Written in Java, Castaway, Demonburps ST Emporium, FAST, Hatari, NoSTalgia and PowerST, PaCifiST, SainT, Steem Engine, STonX
Atari 800 Emulator for the Mac
Macintosh port of David Firth's Atari800 emulator.
Atari++
An open source 130XL, 400, 400XL, 800, 800XL, and 5200 emulator for Unix and Windows.
Atari800
An open source 800, 800XL, 130XE and 5200 emulator for BeOS/x86, DOS, Linux, TOS, Unix, Windows, and Windows CE.
Atari800Win PLus
Windows port of Atari800 based on the original Atari800Win v2.5c sources.
Rainbow
Shareware Atari 400/800/800XL/130XE/5200 emulator for MacOS and Win32.
Rainbow/Mac
A MacOS X port by Richard Bannister of Rainbow.
The CTH Emulator
An Atari 8-bit emulator that supports Winsock 1, OSS Supercarts and has better Sparta Dos support.