Assistive Touch

This feature first made its way into iOS 5 at beta 3. It's a feature found in the Accessibility section that lets you record a screen gesture and attach a function or action to it. It was designed for people who are otherwise disabled and not able to perform certain functions by doing with one finger what normally requires two or three.

But it's not limited to the disabled. Anyone can make custom gestures for their own use. For example, you swipe left to right to unlock the phone. Well, you could make a custom swipe from right to left to lock the phone.