In reply to the sheep:
Like others I've just brought a cheap Cree XM-L T6 light for £15 quid from ebay. There are various styles of these knocking around (i.e. multiply LEDs etc). I have one with a single LED.
Build quality is good for the money, fairly light, simple handle bar fitting system for the head unit, fairly small battery pack. Battery pack isn't waterproof, so I wrapped in a plastic bag and duct tape before putting it in the supplied nylon case.
Its very bright and suitable for unlit roads and cycle tracks, and commuting. For off road riding I think you'd probably want a second one. Cant comment on battery life yet as I've only run it for a few half hr sessions. Need to angle the light down slightly as there is no control on the spill of light, you get one 'hotspot' of light central to the lens, and a less bright wash of light around the outside. C&B Seen do a diffuser lens which will spill the light in a more horizontal fashion, probably a good idea if you are on road with one of these.
There is an led on the back of the head unit which indicates how much battery is remaining (green, blue and red), which is useful. The only downside I can see is you need to physically unplug the head unit from the battery, or the power indicator led remains on (even when the head unit is switched off).
For the money its seems very very good value, and a great light. Personally I wouldn't waste money on anything more expensive.
Prior to this I was using a single cell Lezyne minidrive front light, which was £45, less bright, with a shorter battery life.
A no brainer, as they say.