I have owned these two Photon MicroLight II's forever. At least 10 years. The red one I got in high school to read star charts with. The white one I got at DefCon from the DIS.ORG crew back in 97 or so.
Anyhow, I keep them with me when I'm commuting in case I need some light to find a puncture in the dark or otherwise need light, with the benefit that as tiny as they are, I can still rely on them as worst-case-scenario backup lighting. The white one sticks out like a turd in a punchbowl with a pretty wide angle. It's useless for anything more than being seen and as a pothole-spotter. The red one dangles from the seat rail and flails around while I ride. Very, very visible.
I snagged some pics.
Shown with my Incite 11i for size. I am using hair elastic bands that I stole from my wife to hold them to the clip in my seat bag, and to attach them to my bike if needed.
Shown on my Diamondback Outlook, since that's what I rode today. I've never had to actually road-test this lighting setup, but I used it in our very dark parking lot and bystanders said it wasn't too shabby.
What do you guys use? Just keep a spare LED headlight and blinkie handy? Some spare batteries? Post 'em up.