Emulators

Bernie ][ the Rescue
An Apple IIgs emulator by F.E. Systems Emulation Technologies. Runs on PowerMacs only.
http://www.bernie.gs/Bernie/

Mac Emu
Allow Macintosh computers to run old games and applications. Includes message boards, chat, benchmarks, and tools.
http://www.macemu.com/

MacWindows
Solutions for integrating Macintosh and Windows.
http://www.macwindows.com/

MacWise
Commercial Macintosh terminal emulation software for Wyse 50,60,360, Viewpoint, DEC, Televideo, and Esprit.
http://www.carnationsoftware.com/carnation/MacWise_Info.html

NoSTalgia & PowerST
Two AtariST emulators by Gérin Philippe for the Macintosh. Requires Mac OS 8, 9, or OS X.
http://users.skynet.be/sky39147/

Richard Bannister's Macintosh Emulation Software
Lots of emulators of computers and console systems that run on the Macintosh.
http://www.bannister.org/software/

VirtualPC
A PC emulator for Mac from Microsoft. Allows DOS, Windows, and Linux operating systems to be installed within the emulator environment.
http://www.microsoft.com/mac/products/virtualpc/virtualpc.aspx?pid=virtualpc