I use Spokelites, in green http://www.hokeyspokes.com/spokelit.htm
Very inexpensive, but do require CR2032 batteries. They put off an alien-green bright glow in the pitch dark.

A guy at work uses a downtube light on his bike. It really isn't very visible coming up from behind him, but does put off a nice glow from the side. My commute bike is heavy & cluttered enough (small frames just don't have much real estate). I think the monkey lights look like fun, but the spoke lites serve the purpose. I also have 3m reflector tape on my fork, crank & seat stays. Actually, I feel safer riding now in the 'pure dark' than I do at dawn or dusk...people generally have headlights on to bounce off of my reflective tape & clothes, and the glow of the lights seems to be more obvious.

Be safe & welcome to the dark!