Windows only: One of the biggest bummers about iTunes is it's lack of hotkey support — HKTunes is a free, open-source system tray utility that lets you define custom hotkeys for basic iTunes functions, such as play/pause and track changes.
iTunes does come with a preset collection of hotkeys, but not only are they not customisable, but they only work when iTunes is in focus (which is super dumb, to say the least). HKTunes is a simple, lightweight utility that lets you define any hotkeys you want for play/pause, track changes, and volume control. It even works with your keyboard's media keys (if you have them), which is nice because iTunes often doesn't recognise these out-of-the-box. All you need to do is open up the program's config window, set your hotkeys, and be on your merry way — as long as it's running, iTunes will cooperate.
HKTunes is a free download, Windows only.