CamanJS is a canvas manipulation library offering many tools for pixel-level canvas manipulation. It can crop and resize images, apply blending modes and a host of filters such as: gamma, greyscale, noise, hue, saturation, etc. Unlike with CSS3 filters, the resulting image can be saved.
Seriously.js is a real-time, node-based video compositor for the web. Inspired by professional software such as After Effects and Nuke, Seriously.js renders high-quality video effects, but allows them to be dynamic and interactive. It even has a visual editor in the works (alpha for now).
A lightweight physics engine, written in CoffeeScript. It works with the canvas, the DOM and WebGL.