http://en.wikipedia.org/wiki/Query_by_humming
Query by humming (QbH) is a music retrieval system that branches off the original classification systems of title, artist, composer, and genre. It normally applies to songs or other music with a distinct single theme or melody. The system involves taking a user-hummed melody (input query) and comparing it to an existing database. The system then returns a ranked list of music closest to the input query.