Drilbert is a short puzzle game with a unique digging mechanic. It is available on steam.

Factorio is a factory management game by Wube Software.

Dungeons & Directories is a small text adventure game that you play in your file browser.

freeablo is an abandoned attempt at an open source rewrite of the Diablo 1 game engine. It taught me a lot, but in the end I never finished it.