Many people are aware of Shazam
, the service used on mobile phones where you can play a song near the microphone of your phone and it will (usually) identify the song after a few seconds and tell you the name, artist, album, etc.
There is a free alternative service that you can download and use on your desktop. It's called Tunatic
and it is available for PC and Mac.
But possible the most interesting website I found that offers a similar service is Midomi
. Midomi is also free and in instead of identifying pre-recorded music you can sing it yourself or hum it.
That's a pretty amazing useful service in my opinion. Can't get a song out of your head that you don't know who the artist is? Give it a go.
Midomi is also available for mobile devices.