This is a fascinating post on Kent's Bike Blog: Building a Retro-Direct Drive Bicycle. A retro-direct drive bicycle has two drive systems, one is the traditional style of pedaling clockwise to move the bike forward. But when you pedal backwards you engage another drive system in a different gear ratio to also drive you forward.
Its a very interesting concept and I'd love to give one of these bikes a try. Not just for the fun factor but also because I'd imagine that pedaling backwards would strengthen leg muscles in the same way that running backwards can improve one's ability to run forward.
There are lots of pics and a pretty good run down on how to actually to do this if you wanted to. Really an outstanding and neat concept.