VirtualBox
No, it’s not a porn site and (as far as I know) is safe for work. This is a VMware alternative that is open source and appears to be free. Has a high memory overhead and as such I am not even gonna download it to try… yet. Gonna have to get a bit more memory first. But then will certainly get and try it. Sounds quite interesting. They are working on a Mac OS X version. Say they have it working in Alpha now. Also say Vista (BOO-HISS) runs fine. Supports most versions (?) of linux and they claim even OS2. 😉 Come to think of it, I believe I have a copy of that around here somewhere. Never run it but would be a hoot to see if it would run on my current box. Will see if I can find it…