Try it out here: Harmony
From the creator:
The whole thing is quite modular so I can keep adding more brush styles whenever I get inspired. During the process I found out that, for some reason (apparently lack of hardware acceleration), Firefox and Opera do not support context.globalCompositeOperation = ‘darker’. This was on the HTML5 spec before but got removed. Just so you know what I’m talking about, this is like the “multiply” blending in Photoshop. Webkit does support it tho. I hope they put it back on the specs and all browsers support it.