iHud — Utilities — Free
You can pop into iTunes and find several dozen apps that will use the GPS in your iPhone to create an accurate speedometer, but most of them have a major problem: You have to look down.
But iHud tries to solve that issue. You open it, and your velocity appears. It reads backwards, but if you place your phone up under your windshield, the reflection will look right, and you won’t have to look away from the road to check your speed.
I’m not sure how you keep your phone from sliding off your dash when you turn, but that’s for the engineers.