You know the content in Public Domain has actually *decreased* in the last few decades? It's appalling. But there are some who put such content matching technologies to Good Use. The fine people at MusicBrainz have a software (Picard), which creates a fingerprint of a music file and then checks it against an online database to identify the metadata. It works fairly well even with poor quality MP3 files, but the DB doesn't seem very complete.