Shadertoy - Build shaders, share them, and learn from the best community.
SHADERed - Lightweight, full-featured desktop tool for creating and testing HLSL and GLSL shaders
GLSL Sandbox - An online GSGL Shader Gallery and live development tool.
KodeLife - Real-time GPU shader editor, live-code performance tool and graphics prototyping sketchpad.
The Book of Shaders - Extensive guide through fragment shaders with live coding examples
ShaderLab - WebGL2 GLSL Editor for the browser.