MediaPurge is a freeware application which provides several tools for managing your media collection.
The program can find duplicate files by comparing file information (size, tags), content (find exact duplicates only), or by using an acoustic fingerprint which allows it to match similar files, like two different rips of the same song. The latter option worked well for us, and ran reasonably quickly considering the work involved, processing 10 songs a second on our test PC.
MediaPurge can also bring order to a cluttered "Music" folder, building new folders based on file names or tags and sorting your files accordingly.
There are automated tag management tools to edit, copy or remove ID3 tags, rename a file based on its tags, or generate and assign tags from its file name.
MediaPurge also includes a sync tool for comparing two media collections, finding and ignoring duplicates and synchronising everything else.
MediaPurge's acoustic fingerprinting and ability to find similar audio files is probably the star feature here, although its file and tag management options are also worth having. The English language translation is poor, though, and while there's generally enough visual clues to help you figure everything out, it can be disconcerting occasionally.