HTML5 Canvas JavaScript Library – KineticJS

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.