The Washington Post covered the MusicBrainz Tagger in their weekend edition (reg required):
MusicBrainz does this by computing a music file’s digital fingerprint, based on its length and acoustic properties, then seeing if it can find a match in a growing online database of songs. The program will then work its magic, even reporting its confidence in less-than-exact matches. It was right in most of our tests, even when it reported that a song’s fingerprint matched only 55 percent of a title in the database.
Just wait until they see Picard… Speaking of which, I’m supposed to be working on that right now.