Speedometer is a nifty US$1.99 universal iOS app that provides a speedometer for cars and cycling, as well as start times, time elapsed, distance, average speed, maximum speed and your altitude.
The app also features a map mode where you can see your starting point and current position, and a share button to email your current position information. In addition, Speedometer provides your latitude and longitude, and an indication of how accurate the GPS information is. A compass gives you your direction in degrees, and more generally, like NW or SE.
I tried the app on a short test drive and found it worked as expected and it provided helpful information, more than most navigation programs provide. Living in the foothills of mountains, I especially liked the altitude readout.
You can set the app for MPH or KM/H, and there is a HUD display so you can reverse the display and put your iPhone on the dash and watch the reflected heads-up display on your windshield. It wasn't very effective in bright sunlight, but would be nice at night or low-light conditions.
This is a useful app for cyclists or drivers who want trip information coupled with GPS logging. It's rich in features, and low in cost. There are some similar, free apps at the app store, like Speedometer s54 free and Speedometer Free Speed Box, but those apps don't have the rich feature set and are more oriented toward keeping you under the local speed limits.
Speedometer requires iOS 5, and is optimized for the iPhone 5 series.