Ever so often since I upgraded to Panther, my Belkin three-button scroll mouse loses the ability to click -- although it still moves the pointer. In the past, assuming something had crashed, I was forced to restart my system. But then I accidentally discovered that pressing Apple-Option-Escape (to bring up the Force Quit dialog) miraculously cures my mouse!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040211152607801