Soupault is an open-source static website generator/framework that works with HTML element trees and can automatically manipulate them. It can be described as a robotic webmaster that can edit HTML pages according to your rules, but doesn’t get tired of editing them.
Spotube is an open-source fast, modern, lightweight & efficient Spotify Music Client for every platform.
static-web-server is an open-source cross-platform, high-performance and asynchronous web server for static files-serving.
Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
SuperTux is an open-source classic 2D jump’n run side-scroller game in a style similar to the original Super Mario games.
Six Degrees of Wikipedia is a website which traverses hyperlinks on Wikipedia to find the least number of clicks it takes to navigate between any of the nearly six million pages on the world’s largest free online encyclopedia. (source)
Starsector (formerly “Starfarer”) is an in-development open-world single-player space-combat, roleplaying, exploration, and economic game. You take the role of a space captain seeking fortune and glory however you choose.