Dimensions is a pretty interesting idea for an iPhone app. It's a set of 3D tools that you can actually use in real life, all put together by making use of the iPhone's hardware in various ways. There's a caliper, a ruler, a couple of tape measures for various sizes, a measuring wheel for even longer distances, and a pedometer. There are a few other surprises, too, including a lightning distance calculator, and a level, both likely included just for fun.
Unfortunately, while the tools work well, the app doesn't actually provide any instruction on how to use them. If you don't know how to use a caliper or a level already, you're going to be on your own -- the app doesn't have a lot of polish or documentation, which is too bad. It'd be great to have a Tapbots-level version of an app like this, with as much thought given to the interface as to the functionality.
Still, Dimensions will provide a serviceable ruler, measuring tape, or any of the other tools when called upon. It's available in the App Store right now for US $1.99.