Compass HD offers a range of tools that go well beyond finding magnetic north with a compass -- something iOS devices have built in. With it you can measure and track magnetic fields, measure acceleration in various forms, and much more. The tools are very well executed and offer some fascinating results you can track on your iOS device.
When you load Compass HD, it will, by default, be set to the magnetometer. You can switch it into a compass mode to track location, or you can use it to measure the magnetic pull of nearby objects. The developer claims it can be used as a basic metal detector -- useful for checking the ground for pipes before drilling -- though we were unable to test and substantiate this claim beyond using it to track the strength of magnetic fields in our offices. It worked well in this way: recognizing sources that should emit a stronger field than others. The acceleration measurement is equally interesting, especially when in a car or on a train as forces change direction.
Whether you have specific uses for the tools provided in Compass HD or are more interested in using them to test your environment, the app is well made and fun to use. The interface is basic without any frills but manages not to get in its own way, which is important in an app like this, and with plenty of solidly performing features, this is a great free app worth downloading.