Blackburn 2'Fer. Lightweight, very bright as a warning light, and easily visible from the sides. Not particularly good as a headlight, however - it's bright enough, but too diffused. I'm using it on the front of my Bell Solar, mostly in flashing white mode. I like it well enough that I may add another to the back of the helmet in flashing red. The stretchy rubber band doodad fits through the air vents, and the 2'Fer clips onto the rubber band.

For now I'm using something I already had, an Energizer single red flashing LED. It's bright enough, just not quite as bright as the Blackburn. I used a zip tie to fasten it to the back of my helmet.