A SUPER AWESOME hand crank dynamo!  when you quickly cut magnetic lines of flux with a conductor, an electric current is induced on the wire. A DC motor can be used to generate electricity, but you have to spin it really fast and rectify the output. Gas generators work like this, and so do the turbines that provide the 110VAC powering our houses. This is called "electromagnetic inductance". Look it up.

This crank generator fits in your hand, and the crank can be folded in. I've reached 13VDC+ volts DC at about 200mA+. I actually overloaded my multimeter on the 200mA MAX setting! There is an internal rectifier, so you don't have to worry about rectifying it yourself. This is a special device that is essentially a DC motor with an internal gear set-up that is allows for you to turn the motor shaft several, several times per full 360 degree rotation.