TankChair - Extreme Off Road Wheelchair
This isn't exactly an extreme sport as such, but it is an extreme piece of wheelchair engineering!
Tank Chair is a Custom off-road wheelchair that can go anywhere outdoors. Conquers Streams, Mud, Snow, Sand, and Gravel, allowing you to get back to nature, and can also climb up and down stairs.
The seat is self levelling so people that are paralysed from the chest down are able to go up steep slopes, and down, with no fear of falling out and low enough to transfer with a board.
I'm not sure it's very good as an indoor chair though, it would probably shred your carpets!