If you’re a musician or music fan and you’ve got an iPhone, check out the Digital Sound Meter (App Store link).
Digital Sound Meter is a app that uses the iphone integrated microphone to capture and display the sound level in decibel.
It displays the average and peak levels with a bargraph and with a numeric LCD display.
It works with the iPhone 3G, but the integrated microphone of the first generation of iPhone is not recommended (use the included headset microphone.)
It would be great if Digital Sound Meter indicated dangerous sound levels, like this DB chart. Too many musicians have hearing loss and a free dB meter that indicated dangerous volume levels might help save a few ears.