I'll put in another plug for the Dinotte 200L, this is a nice, bright light (200 lumens) but also very small, compact, and versatile. Plus, they've got a sale going on at Nashbar ;-) As divingbiker mentioned, they run on 4 (rechargeable, preferably) AA batteries, and will run 2 3/4 hours in bright mode on a single set of batteries.

One of my riding buddies just upgraded from an L&M Vega to the Dinotte, and is really happy with it. I'm currently running an L&M Stella 200N, and the Dinotte is just as bright and a little more flexible as to where you can mount it. On the other hand, the Stella has a little wider beam when you get out in the really pitch-dark places.

Me, I'm extremely happy with the Stella. It was about $30 more than I could get the Dinotte for, but also gives me a 5-hour run time, full-on, on a single charge. I ride a lot at night, and that was a big deciding factor for me, at least...