Easel.js

EaselJS is a library for building high-performance interactive 2D content in HTML5. It provides a feature-rich display list to allow you to manipulate and animate graphics. It also provides a robust interactive model for mouse and touch interactions.

It is excellent for building games, generative art, ads, data visualization, and other highly graphical experiences. It works well alone, or with the rest of the CreateJS suite: SoundJS, PreloadJS, and TweenJS.

A series of interesting demos can be found here.