I use one of these as a seatpost bag light because of the reflector feature. The LED's light up the reflector portion so that it is pretty effective even if it is not pointing directly to the rear. One caveat though: you need to perimeter tape the lens on or else it can pop off. Scotch tape will do. It's not secure with just the indents holding it on. Otherwise it's been a good light.
For the helmet I use one of those Cateye 100's. You can wrap the bungie around the helmet adjustment band a few times and it will be solidly anchored pointing straight to the rear. It's not a really big light but it comes in handy if you have forgotten your main blinkie as it's always attached to the helmet.
Originally Posted by mechBgon
Mine too, and if you don't mind a bigger blinkie, I'm finding that the Planet Bike Blinky 5 is good. The lens is a reflex reflector, so the whole thing will reflect if it's getting hit from the rear with headlights, and the LEDs are surprisingly powerful for a budget-priced blinkie. In fact, the hotspot is a bit brighter than my NiteRider daytime-visible blinkie.
The Blinky 5 is natively a clip-on blinkie, so it goes right onto the helmet's occipital-lobe band.