Songbird, founded in 2006, is a duel web-browser and music library organizer. I've just discovered Songbird, and overall it looks to be a very solid project. Since Songbird is open to developers like Firefox, there are tons of add-ons that allow you to optimize your browser for playback, music management, and social discovery.
If you find yourself unsatisfied with iTunes, this looks to be a great replacement.
They have everything from a lyrics engine, an ad blocker, a torrent downloader, and a music recommendation engine, among others. Not that I was not excited about music already, but this opens up many ideas that I was not aware of.