[josephg]

It'd be fun to make a free version using the same concept by rolling your own proof of work system. You could do something like requiring a unique proof of work to be submitted for each pixel change. To change a pixel you need to generate a random number which hashes to a lower value (when interpreted as a bignum) than the existing holder of the pixel.

The UI for something like that would be great - your client could just constantly generate random hashes and buffer a list of the best values its has come up with. Then the screen could highlight all pixels you can edit. As you wait longer (and generate better and better hashes), you can edit more 'valuable' pixels.

(Of course, that would only work if each hash could only be used once. You could embed the pixel value into the hash's input, but then you'd need to decide which pixels you want to edit before you start generating hashes. And that would be way less fun.)