Ah, the old transparent screen trick. Endless possibilities... Though it doesn't QUITE work like this (via Joy of Tech):
Unfortunately, the transparent screen trick doesn't entail a magical X-ray laptop. It is simply an illusion created by taking a digital photo of your surroundings, and then setting the photo as the desktop background on your screen. The end result is the appearance of a completely transparent display.
20 amazing examples below.
MAKE YOUR OWN: Click through to PhoTut for a step-by-step HowTo.