Sol implemented a raymarching rendering algorithm to draw fractals; they're beautiful! You can see the code on GitHub and more image examples here: https://gist.github.com/solsarratea/6c5166db6c70c5d09dbd4da52025f3e4
A better/faster search interface for the Messages app on macOS.
An extensive, clear explanation of how to write a lambda calculus interpreter in Haskell.
Book Wyrm is a federated alternative to Goodreads that lets you track what you're reading and share it with friends. Try it out here: http://fedireads-test.glitch.me/
A narrative scripting engine similar to Ink (http://github.com/inkle/ink), but more portable and implemented using Haxe.
Brian gives a deep explanation of how Monte Carlo Tree Search (a key technique for game AIs) works. He implemented the AlphaGo paper from scratch, so he should know!
A small, readable, self-hosted Lisp to C compiler.
A nice overview of Vim 8's native plugin support and how to migrate to it from Pathogen.
A CHIP-8 (https://en.wikipedia.org/wiki/Chip-8) interpreter written in Rust.