Pollux tags your music files in iTunes so you don't have to
Pollux is a new Mac OS X application by Chetan Surpur and Shashwat Kandadai that will automatically tag your music files in iTunes. It is capable of tagging the name, artist, album, album art, genre, year, and lyrics for each song, and it does so by analyzing the song's audio fingerprint. This means that it can draw on a database of tag information, and do so very accurately, even if the song has no identifying information at all to begin with.
To use Pollux, you select songs in iTunes, then from the Pollux icon in your menu bar, choose Tag Selected iTunes Tracks. Tagging can take a little while, so don't expect instant results. In my testing I found tags to return in anywhere between 15 and 60 seconds. Tagging an entire library this way could certainly take some time, but since it's all automated, why not? There is also an option in Pollux to automatically tag any new music that is imported into iTunes.
There are other applications that do similar things, but Pollux sets itself apart by being both unobtrusive and free. Pollux is still in beta, so you might experience a problem here and there, though in my limited testing it was solid.