I’ve been experimenting with these: http://superbrightleds.com/MR16_specs.htm.
Specifically the MR16-WLX LUXEON LED BULBS. I’ve had the ‘1 watt: 25 Degree Beam pattern - 5500K Correlated Color Temperature - 210ma current draw @ 12 VDC’ since late last winter. It isn’t quite bright enough to ride 20mph in total darkness, but works ok at slower speeds.. I recently bought the ‘3 watt: 45 Degree Beam pattern - 5500K Correlated Color Temperature - 300ma current draw @ 12 VDC’. I haven’t tested it in the dark yet. I did do a runtime test. I was able to run both bulbs at the same time for 3.75 hours full strength and another 15 minutes with some fade. I decided to stop the test in order to keep from running my batteries clear down. (I run 12 AAs).

I should add that the company (superbrightleds) told me these were not intended for outdoor use, but I thought I would try anyway. I rode once in a heavy snow last winter and many times in rain throughout the summer with no problem. I did put small pieces of tape over the little holes on the front of the light in order to keep water from getting in.