Jailbroken iOS: The lockscreen music controls on an iPhone are good enough for quickly pausing or seeking tracks, but the fact that these buttons are buttons make them impractical for use while driving. If you've jailbroken your phone, LSMusicGestures lets you use gestures anywhere on the screen to accomplish the same thing.
A double-tap on the screen — again, anywhere — will let you play or pause. A swipe left or right will go back or forward a track, and the screen itself will update to show what is currently playing. LSMusicGestures is free as long as you've jailbroken.