Fast, cheap and powerful.
I recently bought a flashlight at Lowe's for $25 that shines an impressive 150 lumens. I notched a piece of v-shaped wood molding lying around the garage, tastefully covered it in black electrical tape and strapped it to the handlebar (may post photo later ...). A length of Velcro tape holds the flashlight in place. With a set of throw-away batteries, my investment was about $30. This compliments a so-so LED blinkie light in front.
The reason I did it this way is that I wanted the light now. Didn't want to fuss with online places that can take weeks. The thing may end up devouring batteries but I plan to buy NiMH rechargeable batteries and charger soon. Drawbacks: It doesn't flash or have a low mode. It's slightly clunky/heavy.
Here are the specs. It's a "Task Force" 3 Watt Cree LED, available at Lowe's. It was recommended at this comprehensive site on bike lighting:
http://nordicgroup.us/s78/flashlights.html. Runs on 2 C batteries. If you go this route, make sure to check the website because there's apparently a similar light that's less powerful and easily confused for this one.