I have two Macs at the office, I have one hooked up to the entertainment system and another that I do all my work on. Previously, I used separate keyboard and mouse for each. Thanks to a tip off by the marvelous Snow, today I downloaded Teleport which allows me to control two computers with a single keyboard and mouse simply by mousing across to the edge of one screen which then switches over to the next screen of the other computer. Brilliant, simple, but oh so productivity changing.
I am also going to try out the cross-platform Synergy at some point, so I can also control the WinPC next to me as well as my two Macs, but it seems the Mac port is incomplete at the moment.