Is the world around you literally very noisy? Trains keeping you up at night? Or, is early morning traffic impeding on your sleep? Then, you might be compelled to check out the WideNoise application for the iPhone, an application which allows you to measure the level of noise pollution that engulfs you on a day to day basis. Widenoise has used a sound meter to calibrate the microphone against pink noise---the form of noise similar to the sounds that our ears perceive the best---and then compensates for the limitations of the microphone on the iphone.
To get a sense of the noisy world you live in relative to other Widenoise converts, use the application to map your noise exposure. Checking out the Widenoise map on their website, I can clue myself in to the hum of a sleeping cat outside Milan.
Though Widenoise isn't a necessary addition to your phone capabilities, it may better illuminate your surroundings.