If it's just to be seen, I use a Nashbar Brilliant II. It's pretty bright and a couple of times in a pinch I've used it to get home by on a pitch black night. Not ideal at all but it can be done. they go on sale for < $20. I've had it a couple of years. They still sell them but there's probably better out now.

OTOH, I have a Planet Bike SuperSpot and it's useless. I rode with it ONCE and tossed it in the spare parts bin. What a waste of $30.