I run two on the commuter, one flashing DangerZone on the back of my helmet, and one battery-powered trailer light I wired up myself, very noticeable and bright, and very easy to see from a distance. On the road bike I run a PDW Red Planet on the seatpost, not as bright as the light on the commuter, but I'm moving quicker on those days, so I don't worry as much about it.

Here's the light on my commuter. I prefer a big, steady light that makes it easy to gauge distance from a car to the bike vs a bunch of small lights that flash.