I’ve found the power jumper on Arduino (Diecimila, NG, etc.) comes in handy for quickly powering down a project – helpful when you’d rather not disconnect the DC or USB power. Even handier if you actually replace the jumper with a little slide switch, like so –

Arduino Ng Switcheroo

And who can resist the super-bonus leftover jumper! Well, it’s good to have an extra around anyway ;)

Arduino Crop
Arduino Diecimila