I recently bought a Busch & Muller E-Werk hub dynamo powered supply unit to power and charge up devices while I'm cycling (I haven't actually built the bike I'm going to attach it to yet, but that's a different story!).
It's a small device that plugs into a dynamo hub (in my case a Schmidt SON) and converts the AC output to DC at adjustable voltages. It comes with a large number of cables, including a variety of standard USB plugs and DC charger plugs that allow you to power your phone or charge up a dSLR battery while on the move.
At £140, it's not cheap. But then, being made in Germany, this is not a device made by under-paid, over-worked employees in a far-east electronics factory, and for the time saved while not hanging around wall sockets waiting for things to charge, it's very much worth it.
I'll write more about it once I've got it up and running!