Shadertoy - Build shaders, share them, and learn from the best community.
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.
GSN Composer - The GSN Composer is a free web application intended for rapid prototyping and interactive demonstrations.
KodeLife - Real-time GPU shader editor, live-code performance tool and graphics prototyping sketchpad.
Shdr - Shdr is an online ESSL (GLSL) shader editor, viewer and validator powered by WebGL.