MusicBrainz Picard Tagger

If you've had to change the artist on all compilation album tracks to 'Various Artists' so that they'll group together on old iPods that don't support the compilations flag, MusicBrainz' Picard Tagger is the ideal remedy. It's not a one-click process, more like 3 clicks and a double-check, but it works about as well as any automatic tagger could be expected to (bar the odd bug, but it is still in development).

Read the QuickStart guide first: basically you drag in album folders, cluster the tracks into albums/'releases', then look up each album on MusicBrainz, import the album metadata into Picard, then drag the local album onto the imported album (if it didn't match automatically) and manually match up any files if necessary. That might sound a bit complicated, but generally it gets everything right first time, and there's ongoing work on the interface.