Not sure how well known this is, but, have you ever needed to answer your phone (or whatever), and wanted to pause the song you are listening to on your iPod -- but your iPod is in your pocket and it's a pain to get to. Just unplug the headphones instead, and the iPod will pause.
Caution: Pulling on the wire might damage the plug...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061011170048738