I coupled a Lego Technic Motor to a Jensen #75 steam engine to make a crude generator. From there I built a 5V regulator circuit and soldered in a female USB connection to power any USB device. Since I wanted to use it to charge my iPod, I put in a diode and a .5 amp fuse to provide some circuit protection.
This is a good approximation of power is normally generated. Also, this is a good DIY power generation project, though its not very practical.
Step 1: Obtain a Steam Engine
Step 2: Couple the Lego Motor and Flywheel
I experimented with a couple different ways to couple the flywheel with the lego motor. However, just using a rubber band worked fine.
Step 3: Make USB Voltage Regulator
Since I was going to use this on sensitive electronics, I added several capacitors on the input. The data sheet recommends .33uF but I just used 3 .1uF caps in parallel to achieve this. The output also has a cap on it to smooth the output. I found the output is a rock steady 5.025V. I also added a diode at the very beginning to ensure the iPod battery doesn't try to turn the motor. Additionally, there is a 1/2 amp fuse since the max rating of USB devices are 500mA.