Four, this time, because I know I'm going to go today and tomorrow without having another chance, since I'm moving tomorrow and I'm not going to have Internet access at home or work (eep).
Tachometer glove for race driving- I USED to have a zippy car (VW Jetta GLI), and I always had a hard time knowing where the tach was. I thought it might be nice to have feedback that didn't rely on vision (because taking your eyes off the road at certain times can be...dumb is the best word I can find) or sound (because open windows, music, road noise, and other things can distract you from the actual sound of your engine.
My idea was a pretty simple one- you put a pager motor on the back of a glove that fits snugly, so that the motor is held tightly against the bones to get maximum sensation. I'd guess a wireless link (Zigbee or Bluetooth or IR or whatever) would be most appropriate, and the motor would vibrate at a frequency either equal to or proportional to your engine RPM, providing simple physical feedback that is hard to obscure.