The iPhone Light Saber App is completely free. It's called Phonesaber, and you can download it here. Sadly, though, you will not be able to slice a sizzling swatch of Storm Troopers with just this app. You’ll need this, too.
This pocket Saber uses the iPhones accelerometer (how it can tell when you’re moving it) matched with light saber sounds. The external speakers on the phone, then, make it seems like you’re wielding an invisible saber which is arguably even more daunting of a weapon.