The shimano dynamo hub on my bike produces 6 volts AC and 3 watts of power. This is a nominal rating and is ample to charge a USB 5 volt DC device once you rectify the AC voltage to DC. Most other dyno hubs that I've seen/read about (e.g. Schmidt) also produce 6VAC @ 3W.

There are some bottle dynamo from Schmidt that also do 6VAC and I see there's even one that will do 12V (not sure if AC or DC though). Check the Schmidt website for details.

Also for charging gadgets have a look at the E-WERK from Busch and Muller. It seems an ideal solution for when you're actually riding along but what do you do if you need to hunker down a few days due to bad weather or breakdown?

HTH
Cheers
Peter