I *love* my SuperFlash from Planet Bike. It's the best I've ever seen. I've personally been driving down the road, seen a blinking red light, and only about 1/2mi later caught up to it and realized - yep, that's a PB SuperFlash. Amazing.

If you really want to dazzle them, you can grab one of these bad boys:
http://www.aervoe.com/paints_coating...re-single.html
See it in action:
http://www.youtube.com/watch?v=nxbg9xyeXA4

I saw one of these at a conference and I've wanted to get one ever since. I was thinking of strapping it down to my rack on the back of my bike. Unbelievable visibility - these things are painfully bright, 360º visibility, multiple flash modes, multiple charging options, and durable as all heck! They promise you can run them over with a car and have it still work fine. Not to mention fully waterproof. Surprisingly, only $35 each!

Only thing it really lacks is built-in reflectivity. Although the rubber cover is pretty bright, it doesn't do you much good in the dark unless its turned on.