I wanted everything to be as light as possible, because I am into bikepacking (aka. offroad touring) and have to push and carry my bike a lot.
So this is the generator, a very light, but affordable PD8 hub dynamo. I got mine with a mount for disk brakes, in case I need it some day. It weights about 410 gram, which is less than a normal front hub and a side runner dynamo combined. If you twist the spokes like on this frontweel, you won't need to get extra spokes for the hub dynamo.
The next part is the USB-connector. I got a device which convertes the AC-current from the dynamo to the 5V DC, the USB Standard requires. It's very simple and if you manage to solder two wires together, you can build it. You can harvest the USB connector and the capacitors from old motherboards.
The charger uses the same principles, as the commercial devices. Hence you might be able to charge all kinds of devices, depending on your riding speed. I would recommend charging a powerbank when you are slow, because many devices need a minimum current to start charging.
The difference between this device and other solutions is, that it converts the excessive energy into heat. It might theoretically cause trouble at high speeds, but nobody reported any incidents yet. I would recommend to just unplug it, or plug in an energy hungry device. Most of the cyclist won't ever exceed this speed though.
Since I want to save some energy during the day, I have some Powerbanks. A powerbank is just a battery with an USB Port. I built the white one by myself, using two Lithium Batteries and a simple LM317 Voltage regulator.
After wasting a lot of effort into modifying chargers to charge my camera battery, I got myself a Pixo Universal charger. You can charge pretty much every Lion Battery available.
I also built a small USB flashlight. It gives a bright light for about 10 hours.