Welcome to AcoustID!
AcoustID is a project providing complete audio identification service, based entirely on open source software.
It that consists of a client library for generating compact fingerprints from audio files, a large crowd-sourced database of audio fingerprints, many of which are linked to the MusicBrainz metadata database using their unique identifiers, and an web service that enables applications to quickly search in the fingerprint database.
If you have an unknown music file, one of the applications using AcoustID will very likely be able to tell you what song is it, who wrote it, and a lot more. In case AcoustID doesn't recognize the song yet, you can help by submitting it to the database, so that the next person trying to identify the same song will have more luck.
If you are developing an application that needs to identify unknown audio files, you can use AcoustID to help with that. The service is completely free for non-commercial applications, all you need to do is register your application.