Feedback makes engineering fun, especially robotics. What better way to verify your ideas than with simulations that have cool visualizations?
A cool thing to know is that you can do simulations, GPU-accelerated computations and visualizations simply with your browser and without installing any other mumbo-jumbo on your computer. Some of the libraries listed here are like that.
Particle swarm simulation in the browser
A modular TypeScript library for simulating swarms of particles in the browser, with WebGPU-based acceleration support.
Snake locomotion with Rapier
A tiny browser-based simulator for snake-like robot locomotion using Rapier.
Symbolic and numeric robotics
I don't remember why I called this library robot snake, as it has nothing to do with snakes (perhaps because it is written in Python). It's a library that lets you do symbolic and numeric robotics calculations (e.g. forward and inverse kinematics) and visualize the results.