I've always been interested in little physics sandboxes where you could build things, and was blown away by XKCD's machine a few years ago (
https://news.ycombinator.com/item?id=39953514), which is in-turn based on the classic "The Incredible Machine". Building tiny machines is fun, so I decided to build an app that let's you build one. It's less geared around moving balls from point A to point B, but is more of a freeform build; whatever you can dream up.
Essentially, it let's you build simple machines, mechanisms, and simulations, and watch them play out. There's no practical purpose to it, other than it's fun to tinker, explore, and see what's possible.
Try it out, ask questions, and let me know what you think!
Live site: https://scribblerig.josnun.com
Repo: https://github.com/JosNun/scribblerig