Virtualization and Linux--A Primer (Part 2) - page 5
SWsoft's Virtuozzo uses operating system-level virtualization to divvy up server resources and build homes for guest operating systems. This is more efficient and kinder of hardware resources than emulation. Virtuozzo is a popular product, second only to VMWare.
SWsoft sponsors OpenVZ, which is released under the GPL. OpenVZ is on its way to being included in the major Linux distributions. It requires a custom kernel as well as some userland utilities. There is a Yum repo, and it's in Debian Unstable. Building it from sources isn't too hard- real geeks can patch kernels, and there are good howtos on the OpenVZ Wiki.