Draw
Draw is a simple project to make a paint program to let you draw on the screen. Download draw.sb into Scratch and view the video to see how the application is made from Scratch.
Press the green button to clear the page, P to put the pen down and back up again, C to change colour, and the left and right arrows to change the width of the pen.
Here is a whole gallery and a couple of drawing projects on the Scratch website: