Processing.js is the porting of processing language on the HTML5 world.
The Processing language was born based on a great idea: enable programmers with only one line of code to see something on the video.
Processing language is like a Lego game, you have bricks and you can create great works using only a few of them.
Here is a great site using processing: http://numberpicture.com/picture/search
And here is the official processing site: http://processingjs.org/
Note that the header in the web site is “interactive” !