I second the recommendations for both Nightrider and Urban. I have a pair of Urban headlights (unfortunately they don't sell replacement batteries, so if one runs out on a ride I have to swap the whole light) and a Nightrider in back ... and different but equally bright tail lights on all my bikes,. plus a cheap white blinky up front should I forget a headlight or both should fail, and usually a red blinky on the back (I think two out-of-phase flashes is harder to ignore.)

Even if I go out in the afternoon ... I can't be sure that that day I might deiced I feel better than ever before, and want to keep riding ... or that I will ride so far that i can only limp home. In the summer it's not bad, but in the winter it gets dark early, and I'd much rather carry a few ounces of prevention than count on drivers to notice me ... and also, it doesn't take much to topple a bike. Ride over a stick in the road or a big pothole and not only might I break my bike, I might beat up myself. A good headlight improves my chances of avoiding obstacles.