Oculis Labs has two products, Chameleon and PrivateEye, that protect your screen from over-the-shoulder peepers in two incredibly novel ways. The catch that makes this product unique is that it uses eye tracking, something we haven't heard much of yet.
The high end Chameleon product is targeted towards government offices that really need to protect secrets and are willing pay. It works by learning the unique way that a user's eye jumps around while reading, called their "gaze pattern", and calibrates the text on screen to match.
A low end, webcam based version called PrivateEye is just as interesting. It detects your eyes so that if someone comes in the room and you turn away to address them, the screen will automatically blur to obfuscate whatever you were working on. It also displays the face of the person looking over your shoulder in a little video thumbnail so that you can see who's coming in, and that person can see that YOU know he's in the room.