The Ground Control 40 Dev Board is an open-source 40% ortholinear keyboard shell with exposed pin header, designed by Nick Coughlin aka ncoughlin/NC_Developer for rapid prototyping with any microcontroller of your choice.
Features
45 keys (12x4-ish)
split spacebar ortholinear 40% layout
low-profile (Choc v1), hotswap
folded switch matrix to minimize required I/O pins
combined rotary encoder + 5-direction multiswitch in one (Alps RKJXT1F42001)
The row, column and encoder pins are exposed with a 1x20 socket to easily connect anything to a breadboard.
I tested KBDcraft's brilliant Israfel, a 50% unibody split with an adjustable angle, made of LEGO. It's impressive and offers a huge bang for your buck. Flock to the store, folks!