I don't have a link handy, but the coolest jump starter I ever saw literally jumps starts a car using its own battery. Generally, a dead battery will still have enough power to turn on some lights or click the starter solenoid. This gizmo connects to that power, charges itself up for awhile, and then provides enough amperage to start the car. While it obviously needs some power from somebody's car to work, the advantage is that you don't need to carry around another charged-up device.