For anyone who hasn't seen this trick yet, this Hand Generator Car project shows how to use an NXT motor as a hand-cranked electric generator to power a simple car. No NXT or batteries required!
This trick can be used for other things as well, since you have a kind of analog motor controller (continuous control of speed and direction), although it is not very strong or precise. It makes a good control for rack-and-pinion steering, for example. RCX-style motors and wires work better because the motors are smaller and the wires are longer and more flexible.