Now here's something that's just begging to be taken on as a DIY project: GlobalTop's HD100 GPS heads-up display. Basically a PDA-sized box which projects your speed onto the windscreen and hence saves you from actually looking at the dials the car manufacturer thoughtfully provided, it uses GPS to monitor speed rather than any physical hook-up to the vehicle's electronics.
Engadget played their hands over its blushing surface at CeBIT and found that, while great in theory, even under the controlled conditions of the demo booth visibility of the speed readout was poor, especially when not dead-centre.
Their advice is to wait for a review before splashing down $100; my advice is to reach for your toolkit and make one yourself!
Hands-on with GlobalTop HD100 GPS HUD [Engadget]