EaselJS!

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

Phaser is a fun, free and fast 2D game framework for making HTML5 games for desktop and mobile web browsers, supporting Canvas and WebGL rendering.

PixiJS is an advanced, mature HTML5 creation engine with a fast and flexible 2D WebGL renderer.

melonJS is an open source HTML5 game engine that empowers developers and designers to focus on content.

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.