There are plenty of iPhone apps for your eyes out there, but what about for your ears?
Bina's Audiometry (iTunes, $1) is a hearing test app that measures your threshold of hearing for sounds of different frequencies. You can use it to find out the frequency range you can hear. Most people, the developer notes, should be able to hear up to around 15 kHz, however frequencies around 17 kHz and higher can usually heard only be heard by people under 20.
Gadget Frontier's Decible (iTunes $0.99) is a cool-looking analog decibel meter that displays the average, peak and max decibel reading from the iPhone's microphone. While it's not quite as accurate as a proper C weighted db meter, but for a buck, it's pretty darned close. Use it the measure the volume at a concert, loud sporting event or simply to settle a bet about who can scream/whistle/whatever loudest.
Tip: Ryan Kaplan