Surprisingly Simple Arduino Motion-Sensing Project Makes For Some Weekend Fun article tells that Matt Williamson built a tiny, Arduino-based motion sensor that will SMS you when something moves by your desk or into a room. It’s completely open source and the notifications system runs on your PC thanks to a simple Python script.
Why is this interesting? Well it only requires 67 lines of code to perform something quite useful and the hardware is very easy to build as it uses pre-made boards and a little plastic container to store the motion-sensing components. For more details go to mattwilliamson / arduino-sms-alarm project page.