I have created various little toys in JavaScript that you can view in your browser without any plugins.

You might also want to take a look at my JS games.

Boids - a simulation of flocking/schooling behavior (e.g. birds, fish)

3d engine - a real 3d engine. Unlike Google's, it uses z-buffering, so mutually-overlapping objects render properly. The z-buffer implementation uses an optimization I came up with that saves paint operations.

Rollercoaster / Space flight - riding a rollercoaster, or chasing a TIE fighter to the Death Star. This is a pretty complex engine, supporting arbitrary camera and object transformations, and simple triangle clipping. The animation uses catmull-rom splines.

Starfield - flying through a starfield. The 3d effect is a crude illusion (no fancy math).

Raytracer - a raytracer. It's buggy and incomplete, but works well enough to render spheres.

Substrate - an approximation of a material cracking, which produces really nice patterns.

Dewdney's Voters - a simulation of easily-swayed voters. Each voter randomly picks one of its neighbors, and switches its vote to match. Which party will win?

Popsquares - artsy pulsating colors.