Shadertoy - Build shaders, share them, and learn from the best community.
Shader Editor - Android app to create GLSL shaders and use them as live wallpaper
The Book of Shaders - Extensive guide through fragment shaders with live coding examples
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.
The Force - Open source, live coding editor for WebGL