Shazam has the ability to identify music by listening for 12 seconds of the music and then comparing that music with its database of music... and it does it in about 5 seconds flat! Once Shazam has identified the music, it will provide you with the artist, song title, record label, album title and art, as well as the song’s music video on YouTube, if one exists. Through Shazam, you can tag any music you find and save it for later and buy the music through iTunes.
Wickedly fast and accurate it is, however if your musical taste rests outside the mainstream then it won't be much use to you. My thinking is that in order to return results so fast, Shazam limits its database to the most popular songs... about 6 million alsaid. Now stop reading and go get it... its free.