From this NewScientist article:

If you have ever had a song in your head but don't know what it is, you'll soon be able to get the answer just by whistling to your PC.

Bjørn Olstad, research and development chief at Fast Search and Transfer (FAST) in Oslo, Norway, says their new search software can find a tune and name it with as few as five notes to go on. Users can sing, whistle or play a few notes on a musical instrument into their PC's microphone.

If that stretches your musical talent too far, "simply tapping a rhythm can be enough to locate the right file", says Olstad.

The key to the software is its ability to digitise your ditty and then extract a sequence of pitch and timing data that can be usefully compared with actual musical pieces. The idea is that after you've entered your tune, the search engine scours a database of tunes, looking for matching pitch and timing information.

"Timing is a non-precise parameter. Even a pianist cannot play a piece the same way twice. And the pitch of different singers can vary widely," says Olstad. "Our algorithm has been designed to accommodate this fuzziness."

But there's another problem. At the moment there are not any databases of relevant musical information. So FAST, which runs the search engine at www.alltheweb.com, is negotiating with other dotcoms who might want to set up commercial music databases to catalogue tracks they have for sale.

Olstad sees great potential for the program but not just for tracing half-forgotten tunes. He predicts that scholars will use it to compare compositions, and lawyers to settle copyright battles.