I wanted to have a way to charge my phone from by dynamo hub and I was taken aback by how expensive some of controllers were. Most controllers are designed to mount in place of your cap for the fork tube. It's a convenient place, but the problem with this design was that water could easily get in when my cord was plugged in.

I found a nice controller called the PowerBug. It was A LOT less than the other models and I really like how it uses a nice long cord I can route to one of my waterproof bags. It was easy to put on and I use it to trickle charge a 15000mah backup usb power supply. So far it works really well.