What you are looking at is the front end of my '93 Bridgestone XO-2, which I frequently use for commuting. While most of the bike is original, one big change I made was installing a dynamo (generator) hub on the front wheel, and a light to go with it. Specifically, the hub is from Shimano and the light is from a German company, Busch & Mueller. In a nutshell, the spinning wheel sends electricity to the light, thus negating the need for batteries. The advantage of the hub dynamo over the older style tire-driven style you may remember from days gone by, is that the mechanism is impervious to weather and requires no adjustment. The tire-drive units are prone to slippage, especially in the rain, are noisy, and less efficient than the dynohub.
Now, some of you are saying "but the light goes off when you stop moving, doesn't it?" Well, the more observant might notice that the light is glowing (hard to tell in daylight) with the bike sitting still in this picture. That's because the light has a capacitor that charges while you ride, which powers an LED which comes on when you stop. Not all dyno lights have that, but it's a nice feature.
So, how much does this all cost? I knew you'd ask. Since it involves a new hub and building a wheel, it's true, it's not cheap. However, it's not out of line with other high quality lighting systems. Assuming you start from scratch, and have us build you a whole new wheel around the hub, you're looking at around $250 or so as a starting point. This gets you the high end Shimano hub, new double butted spokes, and a good quality, strong rim, as well as a dynamo headlight. Depending on specific options you choose, it could be a little lower perhaps, or higher. But a good investment for the long run. I can just hop on the bike and know I've got light without having to worry about batteries being dead or forgetting my light altogether.
Come on in and talk to me (Tim) about your options if you're interested. We have a demo wheel built up for you to look at and a couple of bikes in the shop that come with a dynohub stock. And there's a good chance that one of my 3 dynohub equipped bikes will be around.