The Beautypi team (Inigo Quilez and Pol Jeremias) has released ShaderToy, a WebGL application that allows to build, live code and share GLSL shaders. ShaderToy is probably based on iq’s Shader Toy.
Some cool shaders:
One great feature would be to have the possibility to embed a shader in a web page like we can do with a youtube video. The site is still in beta stage, that might explain some lags I encountered (tested on Firefox 19.0 and Chrome 25.0.1364.97).