Marek Kordos' page on the theorem. After all, why take our word for it?
Four-Bar Linkage Analysis and Synthesis at the Integration Engineering Laboratory at UC Davis. Everything you could conceivably want to know about the behaviour of four-bar linkages!
A paper on a parallel motion generator using four-bar linkages. In particular, check out the neat keyboard storage mechanism application.
A very cool applet which simulates the motion of a four-bar linkage. It'll let you drag anchor points to modify the output trajectory!