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Hot off the press (thanks Chris!): ServerStore, an iTunes 4 internet playlist sharing database.
I also spotted this one recently: iPod tracks -> desktop. I haven't had a chance to test this yet, but apparently it lets you copy tracks from a connected iPod (right from the iPod playlist in iTunes) to your desktop. This would be a big improvement over something like Podmaster which does the same, but has to be run as a cumbersome and slow seperate application.