Wikipedia on Operating Systems
PearPC-Power PC Architecture Emulator (For Windows and Linux users-Wired)
Emaculation (Mac Emulators for IBM platform)
Bochs's emulator runs Linux, DOS, Windows® 95/98 and Windows® NT/2000/XP
Emulators, Inc. run Classic Apple and Atari Software on IBM Pcs
Emaculation's Forum is a great starting point for Novices
Wikipedia on Mac Os Emulation software
Parallels for Mac and Windows
Vmware Fusion for Macintosh
SheepShaver (MacOS run-time environment for BeOS and Linux that allows you to run classic MacOS applications )
The Lisa Emulator Project for OSx and Windows
Pure Mac (software for Mac)
DOSBox is a DOS-Emulator that emulates legacy OS (great for retro-games)