I think there is something called TuneUp that I used that was really good (but not free). It rips through your library and cleans up all the metadata (and does a pretty good job of getting songs right that have no metadata).
It let you restrict your genres. So for example, if you had Hip Hop, Hip-Hop, R&B, Rap, Gangster Rap, East Coast Rap in your library, you could restrict your genres to just Rock, Rap, Pop, Blues... etc, and all the Rap sub-genres would get reclassified as "Rap" so it's much easier to browse by genre.
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