Steve Hassenplug has done some amazing work over the years, and one of his newer models is no exception. I first saw this in person at NI Week '07, and was impressed with both the abilities of the big green wheels, as well as the novel turning and steering system. In The Green Monster, the front wheels are independently driven to allow differential steering. But in addition, the rear wheels are articulated to the front body, but also driven by the third NXT motor, leading to more power, and a wonderfully fun skidding way of going around tight turns.
Much more impressive than the look or power of this beast (it handles dirt, grass, and other surfaces with ease... I'm waiting for the snow fan-tailing videos), is the fact that Steve has used it as a testbed for coupling GPS navigation with the NXT. Take a look at the videos (from 14 Sep) on Steve's webpage for some impressive (not to mention rather humorous) navigation attempts (PS, Steve's back yard is considerably steeper than it looks in the video). Currently Steve is using RobotC to drive the robot, which is completely autonomous, simply following a set of waypoint coordinates. He's also used pbLua to communicate with the GPS, and there are even some people who have managed this with the stadard firmware (although it's rather unclear right now exactly how that is done).
Now, anybody know where I can get a really small, simple, BT-equiped GPS unit? Serenity needs a captain... :-).