You can use this app to detect some metal items, batteries, electronic devices, magnets, or other items that are affected by a magnetic field. This app can be used as a metal detector for magnetic metals like iron or steel, but it can't detect non magnetic metals like aluminum, gold, or silver. Also most coins that I have tried don't seem to picked up by the sensor.
When you first use this app it is best to try it with something like a 9 volt battery or a magnet to find out where the magnetic field sensor is on your phone. On my phone (T-Mobile G1) the sensor seems to be in the lower right corner of the phone. For really small objects like screws and bolts you may need to move them back and forth really close to the sensor for them to be detected.
Also There are 4 skins for the App, The main skin looks kind of like a Star Trek Tricorder.