Virtualization in Linux: A Review of Four Software Choices (Techthrob.com)

I think the free version is complete.  I've run OS/2, Windows and Linux under it.  The
drawback is that it is somewhat difficult to compile.  I would recommend getting the code from
subversion. The tarball drops do not contain some of the dependencies so their hokey build
system has trouble when you try to point it to versions installed separately.