It depends on your budget, and how often you need to use the lights, and how bright you need to see the road or just need something just to be seen. Currently LED is the leading technology with brightest with least amount of electricity used, although there are HID and halogen that does well, but uses more juice but these are often rechargeable. If your riding is short, and for streets, the cheaper $20-40 LED AA battery powered lights are good enough. If not bright enough get two of them and you can use one as a backup.
Two good brands fit this catagory: Planet Bike and Cateye.
If your commuting longer than 30 minutes and at least 3-4 days a week, you may need to invest in a rechargeable one. I'm currently looking at one about $100 the NiteRider Sol LED. This is bright enough for some basic trail riding as well.
For serious trail riding at night, be prepared to dish out some money, but it will be a wise investment as it really helps you to see. Typically a combination of bar mount light, and a helmet mount so you can see corners or where ever you are looking at. Again, NiteRider seems like a good brand.
Another thing to look into, is how well the lights mount on your handlebar, and if it gets in the way with other devices you have on the bar (brakes, speedometers, etc) It may be a good idea to bring your bike to the shop to test mount it first.
