Synergy is an open source, multiplatform application to share a keyboard and mouse between two PCs. If you're like me and work with two PCs on a daily basis you'll find it a godsend. No more confusion when trying to figure out which keyboard/mouse you should be using!
Engadget recently had a nice how-to.
Great piece of software. :)
There's also the commercial (though it is only US$30!) alternative MaxiVista that takes things even further - allowing you to use an old PC and monitor as a secondary display. It comes recommended by way of Scott Hanselman's 2005 Ultimate Developer and Power Users Tool List.