DMX sounds pretty cool:
Xdmx is proxy X server that provides multi-head support for multiple displays attached to different machines (each of which is running a typical X server). When Xinerama is used with Xdmx, the multiple displays on multiple machines are presented to the user as a single unified screen.So basically you can hook up other computers and have their displays used as if they were attached via another video card to your computer.
Personally I like my dual monitor + (dual) KVM so I can have two monitors and use them both in both windows and linux, but hey, this lets you use your spare laptop (or two) as an extra part of a screen :) From the slashdot article about dual monitors.