Jamie Zawinski has ported his gorgeous screensaver collection to MacOS.
My favorites are Demon, a colorful cellular automaton; CloudLife, a fuzzy variant of Conway’s Life; Substrate, whose randomly growing arcs suggest the street plan of an old city; Galaxy, a simulation of gravitic interactions; Polytopes, which displays one of the six convex regular polychora, rotating in 4-space.
also: Molecule names and spins a random organic molecule (often psychoactive); Polyhedra spins one of the 75 uniform polyhedra (or its dual)
If I were a better programmer, I’d add an option to Demon: on each cycle randomize the cells on the right and left edges. This would fight the strong tendency to settle into stable cycling spirals. It would also incidentally break the continuity of the topological torus.