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
KodeLife - Real-time GPU shader editor, live-code performance tool and graphics prototyping sketchpad.
GLSL Sandbox - An online GSGL Shader Gallery and live development tool.
The Book of Shaders - Extensive guide through fragment shaders with live coding examples
Shdr - Shdr is an online ESSL (GLSL) shader editor, viewer and validator powered by WebGL.