Mac OS X: If you're tired of constantly sifting through your iTunes library, Tracks will simplify the whole process. You can quickly search for, play, scrobble and control tracks right from your menu bar with a host of custom keyboard shortcuts.
Tracks certainly isn't the only iTunes add-on around, but it stands apart from the pack by combining lots of useful features into one simple program. Its primary function is to let you switch tracks without opening up iTunes and scrolling around your library. Just hit Option+Esc (its default shortcut), search for a track, and hit Enter. It will start playing that track (as well as the search results that came with it) in a new playlist. All the while, it will scrobble to Last.fm, send notifications to Growl, and let you edit many global keyboard shortcuts for controlling iTunes, revealing the track in iTunes and more.
The one quirk Tracks seems to have is that you can't directly play one of your existing playlists or albums; you can only play the track you select and the other search results that come with it. If you search for an album, it'll play that album, but out of order. Granted, choosing a playlist is pretty quick and simple in iTunes, since you don't have to sift through a big library — and Quicksilver will always do that for you, too. Still, we'd like to see a bit more choice integrated into Tracks, since it does everything else so well. If you have a bit of ADD when you're listening to music (i.e. you like to skip between tracks from different albums often), it's a great time saver that bundles a lot of other nice features with it.
Tracks is a $1.19 download in the Mac App Store.