An operating system, a Linux distribution, a quadcopter, and whatever is on the bench right now. Mostly C, mostly close to the hardware.
Air quality monitoring on a custom board.
A ready-to-flash home server distribution, built with Yocto.
A POSIX-compliant operating system for x86-64, written from scratch.
The blog you are currently reading.
A quadcopter built from scratch, frame to flight controller.