Tiltometer - Off Road Gyro Clinometer (iOS) app for $0.99
Tiltometer is a clinometer (also known as an inclinometer) for your iOS device. It is the most accurate you can get because it uses the gyroscope if available, in all orientations!
If you want to know just how steep that hill is, this is the app for you. Great for any off road journey in your 4x4 truck or car.