Shazam is an iPhone application that allows you to identify the name of songs playing. So if you're walking down the street and hear a song in a passing car, use Shazam and your iPhone to listen to the music through the microphone. It will tell you what music is playing.
What’s new in version 4.8.4
Updated on Jan 12 2012
Version 4.8.4:
Enhancement to recognition quality, for more successful tags!
Restores the ability to tag music being played via your iPhone. Really sorry about this, we were trying to fix something else. It's not easy being blue.
Really amazing! It works about 99% of the time for me. Doesn't work too well with live music, which makes sense, but sometimes you get lucky with it. Also detects video using youtube... one of the more powerful and useful iPhone apps out there.