Rough.js is a canvas based, lightweight library that lets you draw in a sketchy, hand-drawn-like, style.
Rough.js defines primitives to draw lines, curves, arcs, polygons, circles, and ellipses.
gaussian, grayscale, highpass, invert, laplacian, prewitt, rgb, roberts, saturation, sepia, sharpen, sobel, thresholding, lowpass 3x3, lowpass 5x5
It is also completely library independent and can be safely used next to NodeJS, jQuery, YUI, Scriptaculous, MooTools, etc.
Fabric also has SVG-to-canvas (and canvas-to-SVG) parser.
Atrament is a lightweight library that enables the user to draw smooth, natural drawings and handwriting on the HTML canvas.
canvid is a tiny dependency free library for playback of relatively short videos on canvas elements.
StackBlur.js is a fast, almost Gaussian blur for canvas. It's based on implementation of StackBlur algorithm.
Two.js is a two-dimensional drawing api geared towards modern web browsers. It is renderer agnostic enabling the same api to draw in multiple contexts:
jQuery.eraser is a plugin that makes an image erasable with mouse or touch movements.
This plugin replaces the targeted image by an interactive canvas that can be erased using touch or mouse inputs. You can specify a callback for completion and set the brush size.