MUST BE IN TURBO MODE! SHIFT CLICK THE GREEN FLAG! Adjust the effect, mode, and size to make some pretty nifty designs. You can also change the settings as it draws the design. Make sure to go in full screen!
As of 5/11/14 there is a new option! I used a sprite that acts as a pen and goes over each and every pixel and sets the color based on x and y values and the effect setting. Make sure to leave credit in remixes!