If you aren't running a dynamo hub, and you're a commuter or you ride at night, buy one. (There's plenty of pre-built wheels.) No need to be afraid of the dark. No need to use batteries when you can generate the required 3-4 watts quite easily.
To go with a dynamo, there's plenty of bright LED lights. I got a spiffy German one, the Lumotec IQ Fly, which is maybe the best one for under $100. I tested it out riding some dark roads in Seattle and the Burke Gilman Trail. Add a LED tail light for $35 that'll never burn out.
Here's a picture I found on Flickr:
Cars have alternators. Bicycles (for transportation) should have dynamos.