SizzlingKeys addressed one of the most painful parts of my transition from Winamp to iTunes: it adds configurable global hotkeys for music control (as well as a few bonus hotkeys for locking or sleeping the system). It's got a clean, simple interface and has been an indispensable sanity saver at work - letting me keep music on and mute/pause it at a touch when calls come in.
--
EDITED TO ADD: Since I'm on a buying spree today, fish owners should check out the Screwcumber. (Hey! Get your mind out of the gutter. *thwaps your nose with a rolled-up newspaper*) Nice solution to the problem of how to sink fresh vegetables to the bottom of the tank.