OK, this one probably isn't going to be helping any amputees any time soon -- but it's still a pretty freakin' sweet concept. Lego and robotics enthusiast (and webcomic artist) Dave "Diavo" Voltaggio has been building himself a functional robotic Lego arm.
He's been working on the concept for over a year, and is currently up to Mk VI: a construction of Lego, Lego Technic, and Mindstorms that fits over his own right arm. Where Voltaggio's hand sits is a series four buttons, each attached to a motor. When Voltaggio touches one of the buttons, it triggers the motor on the corresponding finger of the hand (three fingers and a thumb).
The whole thing is powered by a Mindstorms EV3, and blue LEDs and a phone mount on the inner forearm provide a little bit of decoration (as well as functionality -- Voltaggio can operate the phone using only his left hand). It took a few evenings to build, on and off, and about an hour to program -- not including the time spent on the previous models perfecting the design, of course.
He recently took it for a test drive at BrickFair Virginia (July 30-August 3), where his creation was awarded the Best Mindstorms Brickee award.
If you want to make one of your own, though, you may have to figure it out on your own -- Voltaggio has not yet written down any instructions, although he has offered a few design basics in an interview with Beyond the Brick.