[ilyay]

I put together a similar site a few weeks ago: http://www.livefiddle.com/

It's not as feature rich as jsFiddle or JS Bin (it only allows editing the javascript, not HTML or CSS), but is different in that it's geared towards canvas animations and games. You can edit an init function that gets called at the beginning of an animation, but the rest of the code gets executed inside a draw loop, allowing you to modify code as the program is executing.

Another difference is that you can modify magic numbers in the code using a GUI. Selecting a number brings up a slider that allows you to change it, and selecting a color brings up a color picker.