I like this portable battery-powered security light

The Lumenology Portable LED Motion Sensor Light ($30) is powered by three AA batteries. It has a light detector and motion detector, so it shines only when it detects motion at night (saving the battery charge). It comes with two different mounts: one is magnetic and the other is a flexible tripod that can work as a regular tripod or be wrapped around a pole or a branch. I used the magnetic mount on my front gate and it shines a bright, wide spot of light for 30 seconds when anyone comes to the gate.