Need a battery for your Arduino that is high enough to regulate the voltage to down to a solid 5V? Well here comes the 9V Snap to 2.1mm Barrel Plug Cable to your rescue! It allows you to plug in 9V and 7.2V rechargeable batteries to anything with a 2.1mm Barrel jack on it.
This is the LilyPad Arduino SimpleSnap Board. It's controlled by an ATmega328 with the Arduino bootloader. It has fewer pins than the LilyPad Arduino Main Board, a built in battery, and button snap connectors!