In the classic TV series Exosquad, when humanity set about genetically engineering a slave race that would one day rise up in rebellion against them, they helpfully gave their slaves two opposing thumbs.
Thumbs are useful. They're pretty much the only reason we humans are in charge of this planet and not dolphins. So let's make best use of this advantage by doubling them.
Dani Clode and Tamar Makin of Cambridge University 3D printed a thumb that can be attached to a hand. The motor is strapped to the wrist and the battery on the upper arm. The controlling sensors are attached to big toes. So, if I understand this Guardian article correctly, twitch your toes to open and close the extra thumb.
At a recently science exhibition, Clode and Makin gave 600 people the chance to try it. 98% were able to figure out the control system within a minute, so this is clearly user-friendly technology.
-via Design Boom