A popular way of doing a light around here is to use a halogen down lamp. These run at 12VDC and are available from any garden lighting or indoor lighting supplier. I used a steel can to make a helmet mount and use a 7.5 ah gel-cell lead acid battery which normally gets carried in my Camelback. My bulb is a Phillips energy saver 35W which is supposed to give about 45W worth of light. It is really good and outshines every store bought light I have seen. The battery lasts just over 2 hours so it is good enough for my evening rides in the forest. One detail was to get a headstone plug (odd name, I agree) with a spring clip so it could hold onto the bulb better.

I know most of the systems here are frame mounts which will be fine for street riding but my night rides are typically on single track or forest roads and a frame mount is rarely pointing where I need to be looking so I went with the helmet mount.

As a side note the helmet mount was really complex, made out of several large strips of velcro across one of the vents in the top of the helmet so I could get more surface area connected.

In the end I spent well under half the price of a new system and get better results. I have also made my own brake light/flasher unit for the back but that could be another post, really.