Sinewave Cycles Beacon
This may be an almost perfect light Sinewave Cycles has put together, with only one minor caveat.
It boasts a steady light, even at low speeds, but the most interesting (and possibly unique?) feature is its integrated USB charger. Plug your phone or whatnot into the USB port and you're charging. The switch on the back governs the priority afforded to charging, i.e. how much power from your dynamo is going into keeping the light on vs how much is going into your attached device.
The barrel plug on the back is for connecting an external cache battery. The light will prioritize the dynamo input but at low speeds or when you are stopped, the light will dip into the attached cache battery, meaning that you will always be getting a nice, steady output.
Caveat (only one I'm seeing at the moment) is that the light provides a symmetrical beam pattern. Most dynamo lights out there contain mirrors that produce an asymmetrical beam, which disperses the light in a thoughtful pattern downwards that keeps it from flashing drivers (see Peter White for explanations and pictures of beam patterns for various lights). Symmetrical beams do not angle downwards, and are illegal in some European countries for this reason. This just means that you need to be a bit more careful with how you angle the light on your bike (might make it a better fit for handlebar mounting actually if you prefer that position). Imagine a flashlight beam.
I (Corey) am currently field testing one of these, but it seems like a fantastic light for trail riding (especially at night), camping, bikepacking, long rando rides... or just anytime you need a reliable light and/or a way to keep your devices charged, whether on your commute or out in the sticks. I can't imagine a better designed light for touring. Feel free to email me to pick my brains about it if you are on the fence.
The instructions for the light can be found here.
An FAQ can be found here.
Note: These do not include a mount, which can be purchased separately.