Three.js
threejs.org
The WebGL renderer everyone builds on
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A small live primer of WebGL shader effects
Preview Cool WebGL Shaders inline
Loads the real site in a frame. Some sites block embedding — if it comes up blank, use Open instead.
Eli Chen's browser playground of fragment-shader studies, starting with a ripple. Each page is a running effect plus a short explanation of vertex shaders, fragment shaders and GLSL — useful when you want to see the technique, not just read about it.