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.
Federico is the founder and editor-in-chief of MacStories, where he writes about Apple with a focus on apps, developers, and mobile software. He can also be found on his three podcasts – Connected, Canvas, and Remaster.