Shadertoy - Build shaders, share them, and learn from the best community.
GLSL Sandbox - An online GSGL Shader Gallery and live development tool.
The Force - Open source, live coding editor for WebGL
KodeLife - Real-time GPU shader editor, live-code performance tool and graphics prototyping sketchpad.
SHADERed - Lightweight, full-featured desktop tool for creating and testing HLSL and GLSL shaders
Shdr - Shdr is an online ESSL (GLSL) shader editor, viewer and validator powered by WebGL.