--Emulators--
Sheepshaver:
A very commonly used (and very good) PPC emulator, runs Mac OS 7.5.2 - 9.0.4, runs on Windows, Linux, Mac OSX and BeOS
http://sheepshaver.cebix.net/
Basilisk II:
Another common emulator (68k this time) Emulates either a Mac Classic (which runs MacOS 0.x thru 7.5) or a Mac II series machine (which runs MacOS 7.x, 8.0 and 8.1), depending on the ROM being used, runs on Windows, Linux, Mac OSX, BeOS and AmigaOS
http://basilisk.cebix.net/ (official site)
http://www.emaculation.com/forum/viewforum.php?f=6 (working dl links)
Mini vMac:
Proabley the most commonly used emulator, it emulates a Macintosh Plus with 4mb of ram, runs any Mac OS up to 7.5.5, Runs on pretty much everything, from Mac OSX to Nintendo DS to iPod touch / iPhone, whatever you want to 
run it on there's proabley a port for it
http://minivmac.sourceforge.net/