Want to spice up your car's unlocking system? The Mercuino Project shows off how to unlock your doors with Bluetooth and an Arduino.
The basic premise here is that with this system installed, your doors automatically lock when you walk away from the car and unlock when you get close so you don't have to carry around a key fob. The build uses an Arduino and a Bluetooth board that searches for the phone. You'll need confidence with electronics for this one; you can find the full guide over on The Mercuino Project.