There's a Chrome Experiment called The Hobbit: The Desolation of Smaug that has some pretty fancy effects. While I was playing with it I wondered what open source WebGL stuff people had been making, which is when I found this simple zombie game.
WebGL Debugging and Profiling Tools by Patrick Cozzi has a whole load of resources for working with WebGL. He covers a Firefox WebGL shader editor, WebGL Inspector, Chrome Canvas Inspector, Google Web Tracing Framework, and more.
He even includes useful performance tips:
Depending on how many frames the GPU is behind, a better practice would be to do all the texSubImage2D calls, followed by all the reprojection draw calls, or even move the reprojection draw calls to the end of the frame with the scene draw calls. The idea here is to ensure that the texture upload is complete by the time the reprojection draw call is executed. This trades the latency of completing any one for the throughput of computing many. I have not tried it in this case so I can't say for certain if the driver lagging behind isn't already enough time to cover the upload.
And is glad to see browsers including developer tools for WebGL:
Building WebGL tools, such as the Firefox Shader Editor and Chrome Canvas Inspector, directly into the browser developer tools is the right direction. It makes the barrier to entry low, especially for projects with limited time or developers. It helps more developers use the tools and encourages using them more often, for the same reason that unit tests that run in the blink of an eye are then used frequently.