A motion-based lock screen app called GestureLock was among the winner's at 2106's Android Experiments I/O Challenge, and it lets you set a specific motion (gesture) that you use to gain access to your locked phone.
You set it the same way as you do any other type of screen lock—hold down a button to record your gesture, shake or move your phone however you want, and then release the button. Next time you want to open your phone, you just need to replicate the movement. There's also a back-up PIN in case you forget your gesture, which is likely to happen to a lot of people at first.
Unlocking your phone with a 3D gesture sounds pretty cool in theory. You could use all sorts of elaborate hand gestures and arm movements instead of doing something boring like entering a PIN or scanning your fingerprint.
But what's cool in theory isn't always cool in practice, and before long, you'll probably be known as "that weirdo who's always waving his phone around." Still, it's worth checking out and having some fun with. It's free on Google Play, but hasn't seen any significant updates since last November.