Javascript - KineticJS

Kinetic.js is a successful attempt at extending the HTML5 canvas element with much needed features such as event listeners (click, dblclick, touchstart, tap, etc), performance-tweaked animations and transitions (16 different types), layers and an generally easy to use API for building web apps and games.

Performance is improved by using a hidden canvas layer:

The backstage layer is a hidden layer that uses a custom canvas context to render invisible shapes for the purpose of high performance path detection and executing shape event handlers. The backstage layer is the key to enabling high performance canvas applications with thousands of custom shapes.

To see the results , have a look at some of the demos.

The API looks simple enough and there appears to be quite a lively community discussing it on the forums. Take it for a spin!