When I got my NXT kit I couldn't decide what the first thing I should build would be. I didn't want to just copy someone else peice by peice, but I was stuch and wanted to build something cool. Thats when I found the NXTWay, a Mindstorms version of the Segway. It works by using the Hi-Technic Gyro Sensor to sense itself falling and regain its balance. Here is a video of one version of the NXTWay:
I plan on building something similar yet different- I call it the SegCar GS. It will work similarly to the NXTWay, but with the touch of a touch sensor it will drop down into a car like state. I plan on giving it two programs: a remote control program to be controlled by my PC, and a program where it just kind of sits there. I will post pictures, videos, programs, etc. when I recieve my order of the Gyro Sensor!