Paper.js!

Processing.js is an open programming language, port of the Processing Visualization Language, for people who want to program images, animation, and interactions for the web without using Flash or Java applets. Processing.js uses Javascript to draw shapes and manipulate images on the HTML5 Canvas element.

p5.js a JS client-side library for creating graphic and interactive experiences, based on the core principles of Processing.

Raphaël is a small JavaScript library that should simplify your work with vector graphics on the web. If you want to create your own specific chart or image crop and rotate widget, for example, you can achieve it simply and easily with this library. Raphaël ['ræfe??l] uses the SVG W3C Recommendation and VML as a base for creating graphics. This means every graphical object you create is also a DOM object, so you can attach JavaScript event handlers or modify them later. Raphaël’s goal is to provide an adapter that will make drawing vector art compatible cross-browser and easy.Raphaël currently supports Firefox 3.0+, Safari 3.0+, Chrome 5.0+, Opera 9.5+ and Internet Explorer 6.0+. »

GraphicsJS is a free and open-source JavaScript library for easily drawing any HTML5 graphics and animation from scratch. It is lightweight and more powerful in terms of out-of-the-box features than the existing SVG/VML based components on the market including the segment's former leaders, Raphaël and Bonsai.

Anime.js is a JavaScript animation engine.

StageXL is an API for HTML5 game and application development that closely models and is nearly identical to the Adobe Flash API. It's built on the Dart programming language and offers an intuitive and complete library for impressive 2D content.