Space Ship One, the first private manned space ship, is known for its unusual fuel. What is it?

SpaceShipOne is an experimental air-launched rocket-powered aircraft with suborbital flight capability at speeds of up to 900 m/s using a hybrid rocket motor. Tier One uses a hybrid rocket motor supplied by SpaceDev, with solid hydroxyl-terminated polybutadiene (HTPB, or rubber) fuel and liquid nitrous oxide oxidizer. It generates 88 kN (20,000 lbf) of thrust, and can burn for about 87 s (1.45 min).