Shadertoy - Build shaders, share them, and learn from the best community.
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
KodeLife - Real-time GPU shader editor, live-code performance tool and graphics prototyping sketchpad.
ShaderWorkshop - Qt based, interactive GLSL fragment shader editor
Shade - Pro Shader Editor - Create shaders on iPad or iPhone with a full-featured node based interface.