Originally Posted by DCCommuter

I use Item 1284R on this page: http://www.led-r-us.com/smlights.shtml for a rear light. It's a truck "marker" light with a built-in reflector. It's about as bright as a car taillight, it casts light in all directions (it casts a red pool on the pavement), it's weatherproof, it meets the legal requirement for a reflector, and it runs on my 12V system. Oh, and it was six bucks.

If I recall correctly it's rated at about 1.8 watts.