I really like this project by Joshua McGinnis. It uses an Arduino and ultra sonic sensor to keep track of the distance the user is from the computer. Then it calculates the size of the font and background color based on that distance. Oh, and it will twitter that distance too! Nice touch!

Arduino & sonar returns your distance away from the computer and it is displayed on the screen. the farther away you are from the computer the larger the text and the greener the screen. the closer you are, the smaller the text and the redder the screen. get within 5 inches and a buzzer alarms. distance is twittered.

More about the Arduino sonar buzzer and PHP twitter thing

In the Maker Shed:
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Make: Arduino