Windows/Mac OS X: TidySongs is an Adobe AIR application that analyses your iTunes library to find duplicates, clean up tracks, and better organise your genres (something that's always been the black hole of metadata with my music).
We've seen lots of automated metadata cleaners in the past, perhaps most notably in the form of the recent, previously mentioned TuneUp, but TidySongs throws a new hat into the ring. The catch: Unless you want to buy TidySongs (for a pretty hefty $US30), you only get one feature: Genre renaming.
Normally we'd pass on covering something so limited for free use, but TidySongs is one of the only such tools I've seen that makes genre management so easy for your metadata, and as someone who rarely if ever pays attention to genre because it is always so inconsistent, it's not a bad little tool to have—if only for that feature.
If you feel like ponying up the whopping $US30 for cleaner MP3s and the duplicate finder, they look like they're pretty solid tools—but then again, you're probably better off just aiming for one of these (mostly free) best MP3 tagging tools.