SheepShaver is a Mac OS 9 emulator for Windows, Linux, and Macs running Mac OS X.
I have got it to work on Vista, using Mac OS 9.01 install CD, and Mac OS ROM taken from a Mac LCII. It also works with the “New World” Mac OS ROM taken from “Mac OS ROM update 1.0” extracted with Tome Viewer. These are downloadable from Apple’s web site.
Running under Tiger, you need a new world ROM. I cannot get it to work under Tiger, or Linux.