A GPS tracker for your car, motorcycle, bike and flying saucer, with data storage and map visualization. Not too hard, but no easy either...
Automated parking system using RFID and Ultrasonic sensors.
A DeLorean security system to prevent uninvited drivers.
A mesh of open devices monitoring railway crossings "open-closed" status.
Internet controlled Hovercraft
Need help parking your car? Look no further. The DIY parking sensor will alert you when you are completely in the space.