SizzlingKeys is a free background app that lets you control iTunes playback with hot keys, even when you're in another application.
SizzlingKeys lets you program nine global hot keys: play/pause, previous and next track, volume up and down, show/hide iTunes, show playlists, search, mute, and the ingenious "Almost Mute," with a customizable Almost-Mute level. The app also lets you assign hot keys for rating tracks, and it will display an optional, temporary floating window (with the track info and art). You can also set SizzlingKeys to automatically launch iTunes when you press your play/pause hot key.
This stable, unobtrusive app is a must-download for iTunes users who like using keyboard shortcuts. Upgrading to the Pro version for $4.95 also lets you use hot keys to skip forward and backward in tracks (by a user-defined amount), toggle shuffle and repeat, and control your system volume.