- Some specific details on using Shadertoy & shaders: https://shadertoyunofficial.wordpress.com/
GLSLbin -- "fragment shader sandbox"
- "A Beginner's Guide to Coding Graphics Shaders" -- Really helpful overview. (Part 2, Part 3)
- "OpenGL ES Shading Language Built-In Functions" -- "reference for the built-in functions of the OpenGL ES Shading Language" (via"OpenGL ES Shading Language Reference")
- "The Book of Shaders" -- great introductory text with interactive example code
- Example code is editable and it's possible to share the code and/or have the result displayed in "artwork" mode. e.g.: