Literally Canvas is an extensible, open source (BSD-licensed), HTML5 drawing widget that currently supports a minimal set of drawing operations. You can draw, erase, set the color with the eyedropper, undo, redo, pan, and zoom. It depends on jQuery and Underscore.js.
The instance at the top of this page uses the resizable feature of jQuery UI for its resize feature, to demonstrate that you can resize the container element with impunity. Resizing on the right edge of the canvas causes weirdness, but you get the idea.