Harmony is an HTML5-based online tool that lets you draw on a white canvas using different brushes. It's nothing more than an experiment but it's great nevertheless. It supports a very basic multiply blending effect (as seen in Photoshop) and the developer says it's very first attempt to reproduce the functionalities of a drawing application after playing with the <canvas> element for some weeks.
Also, it works best on Webkit as Firefox and Opera don't support context.globalCompositeOperation = 'darker'. You can save images to .png format.
An experiment, but after seeing Sketchpad weeks ago, we can't deny that the future of webapps is very bright and promising.