Paper.js is an open source vector graphics scripting framework for working with the Canvas element. According to its website, it's based on and largely compatible with the Adobe Illustrator scripting environment Scriptographer.

It's comparable to Raphael, a JavaScript library for vector imaging using SVG. According to the Paper.js FAQ "We have decided to use the Canvas object as the main backend for now because it is faster than SVG and allows us to implement and optimize our own Scene Graph / Document Object Model. We will be offering SVG (and hopefully PDF) importing and exporting in the future."