Probably the best (and also the most expensive) generator you can find. It advertises low resistance when turned off, good efficiency, long life (5 year warranty) and no maintenance (no way to get inside, it's all pressed together). So far after four years of troublefree use (over 10000 km) it seems to be true.
The cartridge bearings are well sealed, there is even a pressure compensation device to prevent moisture ingress during temperature changes. The axle is hollow (solid variant doesn't exist) and the skewer comes with a quick release lever, Allen head (pictured) or a security head. Two output terminals are located at one end of the axle; connectors are included (together with heat-shrink insulation), spare ones are readily available at normal electric shops. Other side of the hub may contain a disk brake mount: ISO (six bolt flange, pictured) or Centerlock (splines). The whole hub is very small and light - lighter than a combination of standard hub and a tyre-driven dynamo.
Number 28 in the name says the generator is designed for 28" wheels. I laced it into a 20" folder where it spins faster and generates more electricity. If you prefer low resistance over high power, Delux variant is a better choice for 20" wheels.
Measured with 20×1,75" (47-406 mm) wheel, circumference 1546 mm was set in the speedometer. Rescaled speeds for 28" wheel are included in the plots. For wheels of different size, divide the speeds by circumference of my wheel and multiply by circumference of yours.
Open circuit voltage rises mostly linearly and reaches 50 V at 25 (34) km/h, I didn't measure it any further.
Maximum short circuit current is 540 mA regardless of speed.
With a 28" wheel and a load of standard light bulbs, nominal 3 W are produced at about 20 km/h. With lower currents the efficiency is better and you get almost half watt more. With a smaller wheel you get a mobile nuclear power plant :-).
For the best possible efficiency, equivalent resistance of the load should be 30 Ω or more. With a 20" wheel, you can raise the voltage up to 12 V and connect LEDs in long series with small resistors (except those which should work at low speeds). For 28", something around 8 V is better.
Source data to download (XLS, Excel 97).