It certainly isn't the first robotic arm made out of Lego that we've seen, but this particular one can actually be worn and easily controlled by the user. According to its creator, Diavo Voltaggio, the arm was made using Lego Mindstorms EV3, which is a software-and-hardware kit for DIY robots. He says the machine is pretty straightforward: it has four buttons connected to four motors that control each finger. Pressing a button flexes the corresponding finger, while releasing it relaxes the finger, as well. Voltaggio calls this cyborg limb the Mark VI, as it's the sixth one he's built, thus far -- he disassembled the first five, so he could learn from them and make better models going forward. As a result, it's much sturdier than its predecessors, though it's also much heavier with all its braces and battery pack.
The Mark VI has won Best Mindstorms creation at a recent BrickFair Lego convention, where Voltaggio demonstrated what it can do by shaking a lot of people's hands, as you can see in the videos below. Its creator plans to build more iterations for future BrickFairs, though, so you might still get to see the arm for yourself.