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.
When there weren't so many options for Linux virtualisation this was where Linode started out: <a href="http://www.linode.com/wiki/index.php/Linode#What_makes_a_Linode_different.3F" rel="nofollow">http://www.linode.com/wiki/index.php/Linode#What_makes_a_Lin...</a>
We used that for quite a few years, were able to get 40-50 VMs on a Dual-Xeon box with 6GB of memory with no trouble. It's very lightweight and flexible with memory compared to systems like KVM and VirtualBox
UML brings back good memories from OS classes. We had to patch ext3 to add some triviality, and I did it first in UML so I could more easily test and make sure it was working correctly. It felt a lot faster and easier to setup than virtualbox or qemu (my options at the time), although I guess with hardware support and things like hypervisors it's no longer practical.
There even exists a port of UML for Windows: <a href="http://umlwin32.sourceforge.net/" rel="nofollow">http://umlwin32.sourceforge.net/</a><p>And that project (indirectly) led to coLinux (<a href="http://www.colinux.org/" rel="nofollow">http://www.colinux.org/</a>), which runs on Windows at the kernel level. Linus himself called coLinux a dirty hack!