Rough.js is a light weight (~8k), Canvas based library that lets you draw in a sketchy, hand-drawn-like, style. The library defines primitives to draw lines, curves, arcs, polygons, circles, and ellipses. It also supports drawing SVG paths.  Demos

I'll go over different code implementations of brushes so that you can understand how to implement free drawing on canvas yourself

Simple pencil, Smooth connections, Edge smoothing with shadows, Edge smoothing with radial gradient, Fur (rotating strokes), Brush, Fur, Pen and etc

First real crack at making an HTML5 game using JS and canvas element.

A jQuery plugin for assisting in the creation of an HTML5 canvas based signature pad. Records the drawn signature in JSON for later regeneration.  Demo

html5 audio visualizer with audio play controls  Demo

An easy to use HTML5 JavaScript Charting library built on Canvas element. Charts can render across devices including iPhone, iPad, Android, Windows Phone, Desktops, etc. This allows you to create rich dashboards that can work across devices without compromising on maintainability or functionality of your web application. Graphs include several good looking themes and are 10x faster than conventional Flash / SVG based Libraries – resulting in lightweight, beautiful and responsive dashboards.

an image pixelating filter jQuery plugin using canvas  Demo