The Book of Shaders - https://thebookofshaders.com/
GLSL Sandbox - https://glslsandbox.com/
KodeLife - https://hexler.net/kodelife
ShaderToy - https://www.shadertoy.com/
ISF - Interactive Shader Format
ISF Video - https://github.com/Vidvox/isf.video
editor.ISF.video - https://editor.isf.video/
gl-transitions.com - A collection of GLSL transitions
OBS ShaderFilter plugin uses HLSL shaders - https://obsproject.com/forum/resources/obs-shaderfilter.1736/
UberViz - https://uberviz.io/
https://learnopengl.com/Getting-started/Shaders
An introduction to Shader Art Coding