National Geographic has made a great video on how Zcorp uses rapid prototyping to make an adjustable wrench. This video explains that RP is an additive process, and shows step by step how it is made. Great video for the classroom if you get youtube! I like the fact that it is a great overview for rapid prototyping: Goes from idea to scanner to printer.
For more info on Zcorp go here.
Some people think that this technology is so far fetched, that Snopes actually had to confirm that it was real!