I looked up the USB standards online and I found that they all use 5V supplies with a minimum of 500mA current. Perfect, I can find myself an old motor and design/build a voltage regulator with a USB output.
You will need: Any permanent magnet motor, a bridge rectifier, one 10 kilo-ohm resistor, any 5.6V rated zener diode, a BC108B transistor and a female USB jack.
Above is the circuit diagram for my design, I tried to make it as simple as possible.
The circuit handles any voltage input type, which primarily depends on the motor (remember to use a permanent magnet motor else this will not work). Any negative voltage is removed by the bridge rectifier and the zener diode keeps a constant voltage of 5.6V across it. The resistor dissipates the rest of the voltage while simultaneously regulating the maximum current that can flow through the transistor from C to E (500mA max). We always assume that the voltage drop across B and E is 0.7V so the output voltage is just below 5V, perfect for charging any USB device.
To the left is a diagram for a female USB jack facing outwards, pins 2 and 3 are redundant in dumb charging operations and can be ignored. Vcc can be connected to our 'USB +5V' output and ground can be connected to our 'USB Gnd' output. Connections to the motor wires are arbitrary.