This app makes use of the gyroscope in the iPhone 4.
The first tab shows the raw gyroscope values in terms of acceleration and attitude. Each value is represented by a graphic to help visualize the data.
The second tab shows you a 3D representation of an object as it responds to combined Gyroscope + Accelerometer movements. It moves along all six axises.
The third tab is an RPM meter. It will tell you the current RPM, average RPM (over a period of time), and the approximate number of total rotations.
The fourth tab is a compass based on the gyroscope. It is much more smooth and accurate and (once aligned) is not affected by disturbances in magnetic fields.