Basilisk II seems to be the undisputed king of Mac Emulation right now. I run
BasiliskII for WinNT/Win2k off of my P3/500 and get decent speed when emulating a Quadra 650. Yes, you do have to have a ROM image off of a real Mac, but those can be found by doing a search on Google. That's how I found mine.

The Java edition is supposed to be faster, but I haven't tried it yet.
Keep in mind though, it can only emulate a 68k machine, so most of the recent software won't work, as it's designed for a PPC-based Macs. There is an emulator in the works that'll supposedly emulate a PPC, but that hasn't been released yet.
I don't run the emulator for anything other than kicks though. And to say I have most of the most popular OS' on my computer.

(Win2k, WinNT, Win98, DOS, Linux, BeOS, MacOS)
Here's a screenshot of Basilisk running. I've successfully managed to connect to the 'net via my cable modem and have some software. It took me a while to find MS Internet Explorer 4.01, but it turns out I had a CD with it on it from a long while back.