Trapster uses GPS to alert you about speed traps, enforcement cameras, and other potential road hazards along your route. Trapster is an essential in-vehicle app for me, but it's exponentially better if your iPhone is mounted in your vehicle. (I reviewed the mother of all iPhone vehicle mounts -- ProClip USA -- back in October)
Trapster 4.5 features much improved map caching and real time traffic overlays courtesy of NAVTEQ. It also features an on-screen speedometer and speed limit signs with visual notifications of speed limit changes:
The new speedometer (above) provides visual notifications when your speed exceeds the posted speed limit. The icon turns yellow at 1-5 MPH over the speed limit and red at 6+ MPH over the limit. Trapster 4.5 also displays a blue notification bar prominently at the bottom of the screen when the speed limit changes.
Trapster currently has over 14 million users and is available for free in the App Store.