I just bought 5 of these little guys for the department bikes:

http://www.niterider.com/prod_minewt350.shtml

The Niterider MiNewt 350. I tested one while I was on midnights this Summer, and I was impressed. The little light puts out a lot of light, weighs next to nothing, has a quick and easy mount (basically, a rubber O-ring that goes around the handlebar) and a nice little lightweight rechargable battery pack that you can either plug into a wall socket or into the USB port of your computer.
Supposed to last 1.5 hours on a charge.
These are compact and light enough that you could put one on your helmet as well. (If you have enough money to afford two!)

These are head and shoulders above the old Niterider units we had years ago, with the bulky water-bottle sized battery and rather short life-span.

I also bought the little rechargable "flea" rear lights. These also plug into your USB port with a little charger that attaches magnetically, or you can recharge them directly from nearly any battery.
Very compact, bright... Can't ask for more.