Two facts: The iPhone's microphone is fairly decent as far as cellphones go; and there are lots of people who can't hear very well. Hence SoundAMP, an app that amplifies and filters noises for ears that need a little help.
The app takes sound from a mic—be it the built-in, in a headset or standalone—and amplifies it according to your demands: you choose the level, as well as which tones to jack up the most (useful for isolating a particular voice, or eliminating a constant background noise). The app also stores the last 30 seconds of audio in a temporary cache, in case you missed anything the first time around.
"But," you might say, "all this app does is pump up the volume on a microphone, which is probably the simplest possible idea for a non-novelty app, ever." And you'd be right! But it's useful in the same charming way that your DSLR serves as an improvised monocular, or your in-ear headphones work as earplugs when disconnected. Plus, it's not difficult to imagine situations in which the app could be useful for non-hearing-impaired folks. One issue: it's $10, which is too much even with the semi-medical pretense. [Techcrunch]