Sketch.js is an open source javascript library which offers some animation utilities. Aimed at making animating with canvas or webgl easier, sketch.js is a lightweight piece of boilerplate code which streamlines a number of common tasks.
Sketch helps by augmenting things like the mouse, touch and keyboard events with often neded utilities. It also sets up your animation loop and helps to manage it with enriched math functions and other handy bits and pieces.
Check out the demo or go and find out more on Github
The project site has good documentation of the API in the project wiki, and many of the conventions and patterns may look familiar to those used to using prototype.