As of January 17, 2010, SmartTag has been marked as inactive. The MusicBrainz project Picard is a exceptional alternative. Download it here: http://musicbrainz.org/doc/MusicBrainz_Picard.
MusicBrainz Picard is a cross-platform (Linux/Mac OS X/Windows) application written in Python and is the official MusicBrainz tagger.
Picard supports the majority of audio file formats, is capable of using audio fingerprints (PUIDs), performing CD lookups and disc ID submissions, and it has excellent Unicode support. Additionally, there are several plugins available that extend Picard's features.... read more
This is a new project so please bare with me while I set everything up. The svn repository is operational, however I will probably create a trunk, branches, and tags structure. In the mean time feel free to checkout the code.