VMware server has been in beta since November, 2007, and you can try it out for free by going to their site. Don’t. It’s unstable and has performance problems - I had to manually reboot our machine at least twice a week because the virtual servers became unresponsive. I originally thought that the problems were caused by the CentOS kernel I was using, but while the problems were lessened after changing it, they persisted.
After downgrading to (the also free) VMware server 1.0.4, the virtual servers have been working like a swiss watch for a couple weeks.