USB - Universal Serial Bus - cunningly allows for a 5v 500mA power supply being fed from the host port, intended for small peripheral devices. This is now commonly used for other purposes, such as charging mp3 players and other gadgets. This is also the same spec as most phone charging supplies, and usb charging cables can be found for most common phone power sockets.
Hence, a source of 5v is often useful, especially if brought out as a usb socket. I suggest any such supply should be regulated, either using a standard or low-dropout regulator IC, or a more efficient cicuit, such as those handily provided within car usb power supplies, often available in pound shops and the like.
The amount of power supplied is 2.5W max; you could charge twenty phones from one bike generator, so there's definitely a case for bulk charging a 12v battery here and using it for sundry jobs, and there is often a relatively long time period involved, so a solar or wind source may be preferable to a hand - wound generator.
It may be useful to note that most devices charged from 5v have internal batteries of approx 3.6 (if 3x NiCad or NiMh cells ) or 3.7v (lithium batteries). Therefore depending on the internal charging circuit you may find a lower voltage may still give a useful charge. As with any equipment, if you exceed the rated input voltage you do so at your own risk; it may well be fine, but it may not.
It may also prove useful to modify other equipment to run on 5v; some small LED lights run on three AA or AAA cells. Measure the current draw at 4.5v, add a little extra resistance to give the same current at 5v and you have a USB powered light. Alternatively modify a device (if you can) to run from 3.6v, add batteries and a charging circuit and you have a USB charged device. Most of this USB stuff is only usefil if you have a charging source available (e.g. a computer), but you may also want to charge a 5v device from other sources.