Via TUAW. It's a free OpenSource app now maintained by Sun. It won't have the support of VMWare Fusion (current leader) or Parallels (contender), but it includes an RDP server for remote access to VMs and it's supposed to support "any x86 based OS" on Windows (least interesting), Solaris (of course), Linux or OS X.
I think this would make most sense for someone with a copy of Windows 2000 who wants to run Microsoft Office 2003 and one or two other compliant apps. (Disabling net access for the VM seems adviseable, though Win2K is probably not a major OS target any more. Who knows, it might now be safer on the net than XP, especially if, like me, you run XP without antiviral software). 
That would be me, except I already have a license to VMWare Fusion. If I didn't, I'd try this.
 Because the antiviral software causes more problems for me than the viruses. I use Firefox with NoScript, stick to good neighborhoods, and use XP as little as possible.