Gargi's delightful RustConf talk explains syscalls and how to write a system call tracer (like strace) using Rust.
A browser-based implementation of the game Plinko. You can see the source code here: https://github.com/alejvillanueva/ComputerPlinko
A tool for teaching web programming that interactively visualizes how the DOM and CSS selectors work. You can see the code here: https://github.com/wylieconlon/dom-simulator
Sher Minn built an interpreter for the long-defuct "ART 1" programming language from the 1960s, and used it to recreate some of the earliest pieces of computer art. You can see the code here: https://github.com/piratefsh/art1.js
A reimplementation of the SQLite database, written once in C, and again in Python.
An open source implementation of the board game Splendor.
A tetris implementation in pure C, using the Allegro library for display.
A real-time chat app that only works if you're inside of an Applebee's restaurant.
A command line program to show you what movies are playing near you!
A thesaurus trained using ML to find words that match the stylistic preferences of a corpus. You can see the code here: https://github.com/kgero/thesaurusx