I was surprised to find a great many effect filters available when setting photos in Leopard's Address Book -- six pages of them! Included are hologram, ASCII art(!), and other great ones. To check them out, just load up Address Book, edit the image in one of your contacts, and click the swirly effects button next to the camera button.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071101114025256