http://www.planetperplex.com/en/img.php?id=94

Good - Evil
In black you can read the word GOOD, in white the word EVIL (inside each black letter is a white letter). It's all very filosophical too, because it visualises the concept that good can't exist whithout evil (or the absence of good is evil - take your pick).

A lovely illusion here of Good and Evil. I think there is lots of potential for an illusion like this used in a presentation.