They're called KVM switches (keyboard-video-mouse) and allow you to run 2 (or more) computers through a single station; they're fairly cheap, and come in a million different flavors. You can get one with just a monitor A/B switch if that's all you really need. I know Black Box corp makes quite a few, but if you look around, you can find a slew more.
Pentium IV 2.4C Northwood- M0 stepping @ 3.2GHz HT
Air Cooled w/ SP-94 heatpipe & 92mm Vantex Tornado at 45C
4x256=1gb dual-channel 2-3-3-6 OCZ PC800DRAM @ 890MHz 3-4-4-8
MSI Neo2-PLS w/I865PE & SATA @ 4x267=1068MHz
BFG GeForceFX 5900@485/967MHz = 6284 3DMark2003s; dual-head 21" CRT & 19" LCD