Here's a cool trick: the new Sonar Ruler app uses the iPhone's speaker to send out a short click and then measures how long it takes for the sound to bounce off a surface. It then tells you the distance.
It's not perfectly accurate, however. It works best on large, flat surfaces that are perpendicular to the phone, which will provide measurements accurate within an inch. It won't work on smaller objects, however, and things like temperature and wind can affect the readings. So you won't want to use it to measure where to cut a piece of wood. But still, that's a damned neat trick for a phone to be able to do. [iTunes Link via Laan via Kottke]