Lego plant watering robot
This robot waters the plants while you're on holiday! You have to line up the plants and through a GUI in Microsoft Access you can input the water demands per plant (how many times per week the plant needs water and how much). The water reservoir contains a mixer and an aeration unit (through lego compressors) to avoid dead water. The only thing not lego about it is the water pump and the robot is controlled by an Arduino Duemilanove. I've used a selfmade multipurpose motor driver pcb so all the electronics visisble aren't used ;)
I'm thinking about making a version 2.0, that actually rides around the house for watering the plants, but that concept still needs a lot of thinking...