Mac OS X only: Freeware application Breakaway automatically pauses and resumes your iTunes music when you unplug or plug in your headphones, perfect for using your laptop in public places. Not only will the auto-pause prevent you from blaring music to everyone around you if you accidentally unplug your headphones, but you also won't miss a beat if you step away with your headphones and then come back and plug in. Breakaway is freeware, Mac OS X only.
Breakaway [via Cool OSX Apps]