Anos: a hobby microkernel operating system written in C
Anos is a modern, opinionated, non-POSIX operating system (just a hobby, won't be big and professional like GNU-Linux) for x86_64 PCs and RISC-V machines.
Anos currently comprises the STAGE3 microkernel, SYSTEM user-mode supervisor, and a base set of servers implementing the base of the operating system. There is a (WIP) toolchain for Anos based on Binutils, GCC (16-experimental) and Newlib (with a custom libgloss).
Anos GitHub page
It's written in C, runs on both x86-64 and RISC-V, and can run on real hardware too (but this hasn't been tested on RISC-V just yet). For the x86 side of things, it's strictly 64 bit, and requires a Haswell (4th Gen) chip or higher.