Try this one: https://gist.github.com/1747707

It basically uses the toDataURL() method to get the image data of the canvas.

well, this method downloads a file without a .png at the end (and most users are overwhelmed with the task of renaming a file)

think there is something coming with BlobBuilder and FileSaver, but i haven't hat time yet to give it a spin http://dev.w3.org/2009/dap/file-system/file-writer.html#idl-... and an example here http://updates.html5rocks.com/2011/08/Saving-generated-files...