While it's not the most groundbreaking hint of all time, it could be seen as handy tip for when you want to take a screengrab and don't want the mouse visible. Simply hold down both shift keys and press the space bar. Voila -- the mouse cursor disappears. It reappears when you move the mouse.
It works in Safari, Mail, and Firefox, but it doesn't work in the Finder.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060712224659954