It wouldn't be everybody's cup of tea, but purely from a "not enough of even their target audience knows about it," I'd have to say the CharaChorder One.

CharaChorder makes chorded input keyboards to increase typing speeds (chorded input is when you press a couple of keys simultaneously, and it autofills a whole word mapped to that key combo - great for long-but-common words).

The CharaChorder One version (as opposed to CharaChorder Lite) actually has a completely different format, and almost resembles an arcade machine's controls if every finger on each hand gets one or two joysticks to control. This obviously nets it a steep learning curve, which is probably a major reason it hasn't picked up steam, but it's great for normal issues like carpral tunnel, or accessibility issues where you have limited hand movement. It also has an ambidextrous mode where you can set all the normal keys to one side of the keyboard, which is, again, great for accessibility, but also if you want to have your free hand on a mouse or... something..... else.......