A self-reference I cannot resist. Primarily a platform for my blogs, but also an ongoing exercise in lisp (the back-end is in lisp), and a bit of rebellious nostalgia for the internet of my youth, when one’s own site would feature one’s own design, from ground up, with hardly a concern for the end user.
A toyish flat spacetime simulator. Fly a ship between the stars, shown as clocks, and see how the time changes according to special relativity. The clocks show the “now” time from the perspective of the ship, which, while not making much physical sense, is technically possible in a flat spacetime. See the associated github page for more detail.