I have really been enjoying the Light and Motion Solite 250 EX.
http://www.lightandmotion.com/bike/solite250ex.htm...
It recharges via USB
250 lumens for 4 hours down to a camp light for 150 hours plus a bunch of levels in between.
It only weighs 149 grams
It comes with tons of mounting options (MTB, Trek, camp, hand held)
Only downsides I see... It isn't fully waterproof and you can't replace batteries (you need to recharge via USB).
$179