So here goes the list. Feel free to suggest others that you've seen around.
Paper.js is an open source vector graphics scripting framework that runs on top of the HTML5 Canvas. It offers a clean Scene Graph / Document Object Model and a lot of powerful functionality to create and work with vector graphics and bezier curves, all neatly wrapped up in a well designed, consistent and clean programming interface.
Processing.js is the sister project of the popular Processing visual programming language, designed for the web. Processing.js makes your data visualizations, digital art, interactive animations, educational graphs, video games, etc. work using web standards and without any plug-ins.
Fabric.js is a framework that makes it easy to work with HTML5 canvas element. It is an interactive object model on top of canvas element. It is also an SVG-to-canvas parser.
Create rich user experinces, animations, games and applications with bHive, a HTML 5 canvas framework API built to make developing easier, so you can start creating straight away.