Wikipedia
Lguest
Lguest is a Linux kernel x86 virtualization hypervisor, introduced in kernel version 2.6.23 (released 9 October 2007). It is a operating system-level virtualization solution, that enhances the Linux kernel with lguest support. Installation is as easy as running modprobe lg and then run tools/lguest/lguest to create a new guest. Lguest is a developing kernel module that in about 5000 lines offers a virtual Linux machine.
Lguest is maintained by Rusty Russell.