I'm writing a minimal, educational WireGuard implementation to practice networking and cryptography: https://github.com/drio/miniwg. It's still in heavy development, but it's been a great learning experience. I've already been able to create tunnels between two miniwg instances and between miniwg and Jason's WireGuard from the Linux kernel.

Before that I wrote: https://github.com/drio/unboxing, a c/webassembly implementation of Danielle Navarro's beautiful IFS fractals.