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
Shade - Pro Shader Editor - Create shaders on iPad or iPhone with a full-featured node based interface.