- https://en.wikipedia.org/wiki/Shading_language - a graphics programming language adapted to programming shader effects (characterizing surfaces, volumes, and objects). Such language forms usually consist of special data types, like "vector", "matrix", "color" and "normal". Due to the variety of target markets for 3D computer graphics, different shading languages have been developed.
- The Book of Shaders - by Patricio Gonzalez Vivo and Jen Lowe. This is a gentle step-by-step guide through the abstract and complex universe of Fragment Shaders.
- Inigo Quilez - fractals, computer graphics, mathematics, shaders, demoscene and more
- https://github.com/mckennapsean/wood-shader - based on the original paper (by Marschner et al.) which established a new sub-surface reflection component to the bidirectional reflectance distribution function (BRDF) that creates a sub-surface highlight distinct to the appearance of actual, finished wood. The key features of this process include identifying the direction/angle of the sub-surface wooden fibers, refracating and reflecting the incoming light ray along this angled wooden fiber, and producing a Gaussian-spread along a cone in the direction of this angled reflection ray. All of these features were coded and ported into GLSL. A sample, rendered image of tiger wood is presented below.
- https://github.com/kbinani/colormap-shaders - A collection of shaders to draw color maps.
- https://github.com/lettier/3d-game-shaders-for-beginners - video_game A step-by-step guide on how to implement SSAO, depth of field, lighting, normal mapping, and more for your 3D game. 
- https://github.com/Chabloom/ShaderTool - a source-level shader translation tool for translating between GLSL, HLSL and MSL (Metal Shading Language).
- https://github.com/mortennobel/sre_glsl_editor - allows you to play with shaders without having to set up everything in OpenGL. The editor is build using SimpleRenderEngine
- Shdr - an online GLSL (ESSL) shader editor, viewer and validator written in CoffeeScript/WebGL that I decided to code while working on HexGL. Indeed, I've been looking for a tool to help me quickly iterate over shader development and didn't find any that would fit my needs.
- https://github.com/dancrn/shade - a quick-and-dirty glsl prototyping tool. it's like shader-toy, but with basically no features of shader-toy. but you can run it off-line, and not in a browser. it has only one dependency - SDL2. this is not intended to be a standalone tool per se, but rather just something that you can easily modify (and extend, if required) to prototype a shader. 
- GLSL Sandbox Gallery - Online live editor for fragment shaders.
- https://github.com/warrenm/Shadertweak - an iPad app that allows you to rapidly prototype fragment shaders in the Metal shading language.