I've had the Phantom Butterflies screensaver installed for a while now. It's a very attractive screen saver, isn't too distracting, and my 14 month old daughter Mishca loves it. She has a good shot at saying, "Butterflies!".
I poked around a bit more and found that White Knight Logic has a few other nice screen savers that are also worth downloading.
Whats your favorite OSX screen saver?