What do you get if you cross a quad-bike with a Segway? This concept vehicle called Centaur:
Concept Centaur combines proprietary dynamic stabilization technology with advanced propulsion and suspension systems, and an intuitive user interface to create a unique four-wheel device that is easily controllable on two or four wheels. Its full suspension and aggressive rider positioning provide an exhilarating ride for one or two people while maintaining control over a variety of terrain. Its rugged performance, zero emissions, and quiet operation make it a good low-impact way to explore the world. Its power and versatility make it suitable for a variety of indoor and outdoor recreational and commercial applications.
http://www.segway.com/products/centaur/ - via Presurfer