Picard is the official tagging application for Music Brainz, an online database that contains information about artists, releases, and songs.
Why is Picard so great? It uses MusicBrainz and that's it's real power, because MusicBrainz handles an acoustic fingerprints database. An acoustic fingerprint is an identifier which is unique for a track, no matter how it's encoded. So, you can get all the tags for a musical track by calculating its ID and submitting it to the MusicBrainz server using Picard.
For a quick getting started guide, see THIS page.
To install MuzicBrainz Picard in Ubuntu, simply paste this in a terminal:
sudo apt-get install picard
For other Linux distributions, Windows and Mac OS X: Download Picard