I used to have paying customers on UML on a Red Hat 7.3 box. I had no idea that UML had been maintained as recently as 2.6.24. I really had no reason to follow it, because UML's niche has been eaten away from both top and bottom.<p>The bottom: For super-high-density virtual private servers, it makes more sense to use OpenVZ (<a href="http://www.openvz.org" rel="nofollow">http://www.openvz.org</a>) and its commercial counterpart, Virtuozzo (<a href="http://www.parallels.com" rel="nofollow">http://www.parallels.com</a>). I've seen installations with 300-500 VMs running on a system with 2G of RAM. It's not a great user experience, but it sure is ... dense. This is where those $5/month VPSes came from.<p>The top: For more conventional virtualization needs, there's Xen and KVM. More flexible, less of a pain in the system administrator's ass. This is the route Slicehost and Linode took.