This was reported on the front page of Slashdot today..... it seems that user-mode Linux has finally been merged into the 2.5 development kernel. To me, UML is easily one of the coolest features of the new kernel series. What is it? Well, from usermodelinux.org:

User-mode Linux is a patch for the Linux kernel which allows an executable binary to be compiled and executed on a host Linux machine. The kernel can be assigned virtual resources, including a root filesystem and swap space, and can have a hardware configuration entirely separated from that of the host.
So what does that mean to you and me? Well..... from the project homepage:

As a secure sandbox or jail

Processes inside the user-mode kernel have no access to the hosting machine or to the outside world that's not explicitly provided. So, a malicious application running inside it can do no harm to anything that matters. Should viruses like those now plaguing other, inferior operating systems ever start appearing on Linux, the environment provided by this kernel can provide pretty solid protection against them.
Conventional wisdom holds that ideally, each service should run on a separate dedicated server for maximum security. But in actual practice, it's often too expensive or too impractical to have separate boxes for web, ftp, DNS, etc. Now you can run each of those services on one box but in separate virtual machines and get the same effect.

Oh yeah.... you know that link to usermodelinux.org above? It's running in a virtual machine.

This is WAY COOL stuff.