There are a few very good hub dynomo lights on the market that have unnoticeable resistance-but they are expensive. Shimano makes one that is probably the least expensive at around $250 plus the light, plus respoking your front rim.
If the dynomo price is too much to swallow, there are plenty of lights on the market that have good run times such as HID units which are far brighter than the dynomo's and last about 6 hours but cost around $400. Again pricey and how much light do you really need? The HID's are too much for the street.
So the other end of the scale are the self-contained battery units such as the Cateye EL300 which is the brightest light of it's kind on the market for around $35; the 4AA bats will last about 20 hours. These self-contained battery jobs are lights that are more intended to be seen not to see with.
The light I bought is a Cygo Metro because it was the lowest cost light I found at around $46 and is more than bright enough for street riding. It has 2 beams, one is a 6.3 watt flood and the other is a 6.2 watt spot or you can run both for a total of 13 watts. The other reason it appealed to me is that it runs off of 6 D batteries that are in a soft case that attaches to the top tube; this means that I am no longer tied to a wall as a rechargeable would be. Also with rechargeables is that if the bat dies it does so suddenly without much warning, the alky bats that I use would take about 1 hour to die and you notice it slowly dimming. The run time on Metro is 5 hours on one beam and 2.5 hours running both. This unit can be upgrade anytime to a rechargeable system by buying the Rover charger and battery then you would have 2 bat systems.
There is a new Cygo called the Rover NiCad 6 hour for about $68. This light is rechargeable only but on 6 watt low it will run up to 6 hours, but combined with the 10 watt high (16watts total) and your battery life goes to about 1.5 hours
When I commute the brightest light I need and use is the 6.3 watt flood and only use the high with the flood when I'm on a pitch dark bike path. I get my bats at WalMart and buy their inhouse brand called EverActiv and get 12 bats for $8 and they last 1 year (of course in the summer months I don't need the light).
see:
www.cygolite.com