Open Source Drawing Widget – Literally Canvas
Literally Canvas is an open source HTML5 widget, which can be integrated into any page, for enabling users to draw online.
It comes with a simple set of tools including draw, erase, color-picker, undo, redo, pan and zoom.
The widget is built with jQuery + Underscore.js and has an API to define the background color, tools and dimensions.
Also, an export function exists for easily pulling the drawn canvas as an image file.