A while back I started a "world's ugliest taillight contest" thread -- I've updated the design to 20 LEDs (100,000mcd total) and two modes and found a better way to package it.
The two modes are compound flashing (all 20 LEDs) or "throbbing" where ten of the LEDs are on steady and the other ten use compound flashing.
As I mentioned in the original thread, the goal wasn't so much to create a light that was effective at night (though it obviously is) but to create safety lights for day riding. This thing is brighter than many automotive eye-level brake lights and the flashing is quite noticeable
It's tedious to build but I haven't seen anything near this bright in any of the bike shops around here. Later, when I get a bit of free time to draw up usable instructions, I'll post a construction article on my website.
You can build two of them (one front and one back) for about 50 bucks less batteries and charger (assuming you already have things like a soldering iron, moto-tool, glue ***, and patience).
I purposely set this up with a separate power supply so it can be run from NiMh cells, Alkaline cells, a generator, a solar cell, or some combination of the above. (The next step is to design a NiMh battery system recharged continuously via solar cells mounted on the trunk with "quick Charge" capability from a wall-wart.)