Niterider 19-LED taillight.
I dropped by the local sports shop and they had an open box niterider digital evolution 12 tail light (their largest cluster with 19 LED's) for sale for $19.95. All the connectors were missing but I figured I could deal with that.
So I bought it, got an 8-AA battery holder from radioshack for $2. Spliced the wire from the niterider and hooked it up to 8 nimh 2500 milliamp AA's. I'm measuring around 10.2 volts from it. Supposedly it's even brighter at 12 volts but it's damn bright as is. Put the battery pack in my saddle bag and I'm ready to go. Draws around 350ma so I figure with self drain and all and only on the night commute, I'm good for a week before having to recharge.
Here's a pic of it with the (clockwise) niterider tail light, a cateye 2-LED blinker, the planetbike helmet mounted blinker, and a specialized blinker. All the white spots are reflective tape.