This is one of the most spectacular applications of Arduino the team at Freetronics has ever seen! The project allows you to use your smartphone to remotely trigger up to eight different fireworks. Not only is it awesome to be able to control fireworks from your phone, but the controller has the added benefit of enabling you to stand well clear of your fireworks when you launch them, drastically boosting safety.
At a basic level, an Android application sends commands over bluetooth to the Arduino controller, which then switches a relay. The relay provides power to the appropriate heating coil, igniting the firework! You can even use the Android application to pre program specific pyrotechnic sequences, so you can be running your own fireworks shows in no time! To get started with this project check out the following instructable from tominjose.
Check out the following video to see the firework controller in action: