A ski heads-up display with a speedometer
Some hackers propose to put sensor under your ski so that you can have a measure of your speed displayed on your goggles!
Using optical mouse technology, you could mount an LED and an optical sensor to the underside of a ski or snowboard to measure the speed you are travelling over the snow. A numerical LED readout could be mounted to the top of the ski or projected in a heads-up display in your goggles for easy reading.
The device could also store your max speed, average speed and total distance travelled (so you could know just how much skiing you're getting in for that incredibly expensive lift ticket).
The comments are insightful and apart from giving a name to this project ("Obvious name: Skeedometer"), they raise numerous questions about it (i.e. surface granularity or calibration). Some are really doubtful and propsoe using a GPS instead of this mouse crap. It's too bad there is no mock-up!