I just got a couple sets of Tire Sparx. They're LEDs that attach to Schrader valve stems (or adaptor). I was so happy with them I just wanted to spread the word. They were about $7 a pair, and I think they make for a nice addition to my lighting (see review here: http://www.kk.org/cooltools/archives/001761.php). In Canada, they're available from MEC.
I got a green set and a red set; the green are much brighter than the red.
I cant buy into stuff that takes offbeat non-rechargeable batteries, but thanks. Do this for a while, and the cost of batteries gets really old really fast.
I bought two sets for my bikes because I thought the idea was great however, after using them three times, they came off. While they are cheap, they require 3 batteries each. Also, they are always on which means if there is vibration around they start to flash. I took out the batteries to shut them off and when tried to reassemble them, they stopped working altogether. Spoke-Lites are a better way to go.
My philosophy is that, if it helps keep me safe, it's probably worth the investment. I'll never know how many collisions I've avoided thanks to my mirror, reflectors, and lights, or how many broken bones I've prevented thanks to my studded tires, but I'll never regret the extra outlay. By the way, my tire sparx were still working today... -35C.
Even with my best efforts, I usually have to take evasive action a few times a week when a motorist doesn't see me. It's dark, they're cold, and they often don't do a very good job scraping their windows, especially the rear, since the defroster will take care of that eventually. Then, their first maneuver is to back out of their driveway/parking spot. Plus, I definitely experience that 'safety in numbers' effect in winter; when cyclists are rare, nobody thinks to look for them.