Processing.js is the sister project of the popular Processing visual programming language, designed for the web. Processing.js makes your data visualizations, digital art, interactive animations, educational graphs, video games, etc. work using web standards and without any plug-ins. You write code using the Processing language, include it in your web page, and Processing.js does the rest. It’s not magic, but almost.
Fabric provides interactive object model on top of canvas element. Fabric also has SVG-to-canvas (and canvas-to-SVG) parser.
Two.js is a two-dimensional drawing api geared towards modern web browsers. It is renderer agnostic enabling the same api to draw in multiple contexts: svg, canvas, and webgl.
Paper.js is an open source vector graphics scripting framework that runs on top of the HTML5 Canvas. It offers a clean Scene Graph / Document Object Model and a lot of powerful functionality to create and work with vector graphics and bezier curves, all neatly wrapped up in a well designed, consistent and clean programming interface.
Pixi.js is a 2D webGL renderer with a seamless canvas fallback that enables it to work across all modern browsers both desktop and mobile