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@TheBrick(Tommy)
most dynohubs (including shimano) output 0.5A. The "standard" bulb to use is 2.4W if you have a rear light and 3w if you have a only a head lamp.
Watts = volts x Amps
So nominal voltage is 6 v, but the faster you go the more voltage and hence more watts are produced.
You blow bulbs because you are going too dam fast for an incandescent bulb! This is easy to achieve mind you. You can make simple voltage regulator with some kind of rectifier to dc that charges a battery and then onto your bulb.
For LEDs the output is is obviously AC so if you want to run a led you will need some kind of rectifier. A simple bridge rectifier is enough. I expect what happened with your LED is because there was no rectifier you surpassed the maximum back voltage (brake down voltage) and it broke down. High power LEDs require good heat sinks though no make sure you have good thermal bonding to a large metal surface.