The other one that you might want to try out is called "Q", which uses the QEMU emulator. Like Virtual PC, it's slow and might be a little unstable for some operating systems, but it's open source which means it's free for use. I have it running on my iMac G5 when I want to test out some PC operating systems like Windows, Linux, or the free BSDs. You can find it at the following link:
http://www.kju-app.org/kju/