Similar to skate boarding, but without the board. It's mostly starting to surface in France and surrounding area and is one of the most impressive sports I've ever seen. It's simply amazing what you can do with a wall, a pair of sneakers and the agility of your own body. Of course you also need a very large set of balls in some cases.
Check out this little video of Free Running, it's pretty impressive. It doesn't start off to spectacular but keep watching and you'll be amazed.
Havoc