Last week I used a Leostick[1] and an SR04 ultrasonic distance sensor[2] to create a presence detector for my computer. Now, if music is playing whenever I get up and walk away from my computer, it pauses the media player, and when I come back it starts playing again automatically.

[1] http://www.freetronics.com.au/products/leostick

[2] http://www.instructables.com/id/Simple-Arduino-and-HC-SR04-E...