Apparently Leonardo Da Vinci conceived of and drew plans for a keyboard instrument that was never built during his lifetime. Or for nearly 500 years after his death. But now the plans have been found and dusted off, and the thing's been built. And so the world now has the "viola organista"
My take on it based on the sound in the video below and the description in the link above -- though I'm hardly an expert -- is that it is similar to a piano. But whereas playing the keys on a piano causes hammers to strike the strings, on the viola organista the strings are pushed against a wheel that is wrapped in horse tail hairs, similar to the bow. In addition, the player has to push a pedal to keep the bow-wheel moving. The effect sounds oddly like a string ensemble.
The video below shows the world premiere of the thing. It's the same video as in the article that I linked to.