We live on a street with no streetlights and it gets very dark at night. People who come to our house at night have trouble seeing things. I bought one of these $(removed) solar powered motion sensor lights with a 20-LED array and PIR (passive infrared) motion sensor and mounted (hardware included) it on a 12-foot wood column at the far end of the driveway. Lucky for me, the solar panel is facing the south so the batteries get a good dose of juice every day.
It works really well. When a car or person enters the driveway at night, the light comes on and throws surprisingly wide coverage. The light is so bright that it hurts to look directly at the lED array. The back of the unit has a 3-mode switch. You can set it to turn on and stay on when it gets dark (in one of two brightness levels) or to turn on only when it detects motion (the setting I use). People on Amazon say the batteries run down in 11 hours if you set it on the dim mode, and 2 hours in bright mode.
Update: Boing Boing reader Joe Stirt recommends this 32 LED model, which also costs $(removed):
