HTML5 Canvas JavaScript Library – KineticJS

KineticJS is a free JavaScript library for working with HTML5 canvas much easier.

Shapes and images can be drawn (there are ready-to-use ones like circle, rectangular, etc.), animated, rotated, scaled and can be set to interact with other shapes or user events.

With the capability of "high performance path detection", it runs fast and can work with thousands of shapes at the time.

P.S. There are also handy plugins built for the library which can be found in the "forums" of the resource.