User Mode Linux (UML) was the first virtualization technology for Linux, also knows as "a port of Linux to userspace".
With everything running in userspace, UML needs a little bit of help from the host kernel. It is also not the fastest virtualization technology around. However, because the UML kernel runs in userspace, User Mode Linux is one of the easiest environments for debugging the Linux kernel and something every beginning Linux kernel hacker will want to experiment with.
You can find more information on http://user-mode-linux.sourceforge.net/