Alex Eames, over at RasPi.TV has been experimenting with a rain sensor. He’s figured out how to use the digital-out of a breakout board to trigger a buzzing alert when water is detected. By tuning the breakout with a potentiometer, it can be configured to detect the first few drops of rain and let you know. That means that your worries about not bringing the washing in when it rains are gone! You can read a tutorial and see the code here and you can get hold of a small kit to do the project yourself here.