The Book of Shaders
Extensive guide through fragment shaders with live coding examples
6 Alternatives To The Book of Shaders
KodeLife
Real-time GPU shader editor, live-code performance tool and graphics prototyping sketchpad.
ProtoShade
Interactive OpenGL Shading Language (GLSL) compiler/editor for Android
SHADERed
Lightweight, full-featured desktop tool for creating and testing HLSL and GLSL shaders
ShaderLab
WebGL2 GLSL Editor for the browser.
ShaderWorkshop
Qt based, interactive GLSL fragment shader editor
Shaderific
Create shaders on the go with the OpenGL shader development app for iOS