The "it" light right now is called the P7. It is a super bright LED (actually a set of LEDs) from a company called Seoul Superconducters. That LED can be put in different kinds of flashlights. The popular cheap ones are those on DealExtreme.com. They run on one Lithium Ion battery, which I believe is a little bigger than a AA. DealExtreme (or DX) is in Hong Kong, but shipping is free (although not always fast). There are some other lights you can buy that also use the P7 LED, but they are more expensive. I doubt you'll see them on Amazon, as the others I've seen are custom made by small shops.

There are lots of threads in the lighting forum here about them, probably a good starting point is this one: http://www.bikeforums.net/showthread.php?t=493600. There is also a very long thread here that dates back to the introduction of the lights, so some of the info in the early posts may be outdated by now.

The setup I bought, which was recommended by others there, is this one (look up the item numbers on www.dealextreme.com).

p7 Light 16092 (supposed to be better than the 13060, even though they look the same)
batteries 5790
charger 6105
mount 8274
mount 15642

Total is about $70 (free shipping). This sucker is supposed to put out more light than a Dinotte, which is triple the cost.

Remember that it is a flashlight, not a bike light, so you'll have to try one of the mounts I listed above or fabricate something yourself. It is the cheapest way to get lots of light.

Runtime is supposedly about 80 minutes on high and several hours (maybe 4 or 5?) on medium (bright enough for most riding).