Back in August, I dreamed about the ultimate downhill road bike. I was assuming that this bike would also be peddled uphill. Say you didn't assume that. Then you'd end up with something like these nut jobs built for themselves:
They're competing in something called the MaryHill Festival of Speed which is basically a gathering of people (mostly young males) wrapping themselves in leather and helmets and then placing their bodies on wheeled objects that seem to me like carbon fiber skate boards and like-a-bikes and pointing them down a big hill.
Here's a pretty good blog entry on the world's fastest gravity bike. CNC'd aluminum components, gigantic custom motorcycle style disc brakes, and shocks to cushion your ass while you kiss it goodbye.
Of course, nothing this crazy goes un-Youtubed. I'm sure you can do some good research there.
Picture from bikeportland on flickr.