MultiseatX
Multiseat X is a where one machine powers several monitors. There are several different ways to do this, as outlined below.
Ubuntu 6.04 (Dapper Drake)
As of X version 7.0/6.9, there is multiseat code built into X. A howto was written by Chris Tyler and can be found [http://blog.chris.tylers.info/index.php?/archives/14-Multiseat-X-Under-X11R6.97.0.html here].
There is also the multiseat package, but it current only works on older versions of Ubuntu.
Ubuntu 5.10 (Breezy Badger) and Ubuntu 5.04 (Hoary Hedgehog)
There is a package written specifically for the HP 441 computers that is available in these versions of called multiseat.
Other implementations
[http://www.tldp.org/HOWTO/XFree-Local-multi-user-HOWTO/index.html Backstreet Ruby docs] - needs a kernel patch and outdated as of X.org 7.0/6.9
[http://userful.com/products/dm Desktop Multiplier] by Userful - closed source