For the rear I use a Planetbike Superflash and for the front a Quark AA2 with a Twofish Lockblock mount and rechargeable batteries. This is a very visible combination and the LED flashlight compares very favorably to purpose-built bicycle headlamps in terms of lumens, cost and visibility.

If it's dark the Quark goes in one of the steady modes (so I can see) and in dawn or dusk conditions I put it in strobe mode (so I can be seen). A couple weeks ago when I had it in strobe due to rain (it's waterproof, too), I pulled up to a coffee shop and a stranger commented that he could see my light coming all the way down the hill, which was about 2,000 feet, give or take. It's probably visible further away than that but there's a curve in the road.