Physics
Simulations that run entirely in your browser. Each one keeps its physics in a plain, unit-tested module, with the page doing nothing but drawing it.
Double pendulum
A chain of one to five rods, and as many copies as you like released from almost the same place. One rod is integrable; two is deterministic chaos.
Vicsek flock
Self-propelled particles that align with their neighbours. Turn the noise down and the flock orders itself out of nothing; turn it up and the order dissolves.
Forest fire
Trees grow, lightning strikes, fire spreads. Nothing tunes the lattice to a critical point, but it walks to one anyway — and the fire sizes come out scale-free.