This is one of the most versatile LED self-contained lights, and at an affordable price. The unit contains four AA rechargeable batteries.
They're NiMh and 1800mh capacity, which isn't bad for a start. It's switchable between full power, low power and flashing modes, and the unit itself is well sealed against the elements. It comes with an adaptor and a desktop recharging unit, ideal for at work or at home.
The light it gives off is just enough to ride with, but the visibility is very good. The LED is not hidden inside an opaque light body - it's left exposed, meaning that all of the usually hidden light spill is allowed out through the side, making this a very visible light.
The nice thing is that the light isn't sealed; you can just slide the battery cover off and replace the four rechargeable batteries with regular AA cells should you forget to charge it up. A solid mounting is a bonus, and it'll fit standard or OS bars but not wing units (unless they have a round section where you can mount it).