With the Sonder’s e-ink keyboard, each key has a tiny e-Ink display that can change on a per-application basis. Available Q4 2016 (that means soon) for $199. Maybe not cheap but still pretty darned cool.
More info here.
The Sonder Keyboard combines a sleek new design with a built-in rechargeable battery and enhanced key features. With an improved mechanical mechanism beneath each key for increased stability, as well as optimized key travel and a lower profile, the Sonder Keyboard provides a remarkably comfortable and precise typing experience. It pairs automatically with your Mac, so you can get to work right away. And the battery is incredibly long-lasting — it will power your keyboard for about a month or more between charges.