Can't find a metal object? Your iPhone can actually work as a Metal Detector and help you out. The aptly named app Metal Detector utilises the compass by reading its interference with metal objects to detect where they are. Although imperfect, it helped a friend of mine find a fork he dropped in the grass at night so if you lose something, the app can be pretty useful.
So how does it work? I have an iPhone 4 and it seemed to have the easiest time detecting metal when I pointed the top (where the headphone jack is) at objects. It could detect from a distance, but was much more accurate upon getting closer. If you're having trouble finding success with the app, you can boost the sensitivity. I found it had no trouble detecting an iPad or a metal vase, but was a little more subtle when detecting a set of three keys. It seems that the metal needs to be magnetic to be detected (eg steal, iron and so forth), but that's only a drawback if you're looking for a type of metal that isn't. If you lose a metal object and all you've got is your phone, Metal Detector is a free way to help you find it faster.
Metal Detector (Free) [iTunes App Store]
Thanks for the tip, Trevor!