Superstar
If you consider yourself somewhat of a mechanic, then you need to check out the Logworks app & OT-2. Logworks has created the OT-2 ('Open Tune') as a portable OBD-II that works in conjunction with the Logworks app, providing you on-the-go automotive diagnostics right on your iPhone.

The Logworks app & OT-2 can be used for checking and resetting engine lights, tracking your 0-60km/h time, and even for managing your gas mileage. Check out a few sample shots of the application in action in the gallery.