Matt Davis has designed the ipythonblocks visualization tool for the IPython Notebook. It uses squares depicted in a browser to help students understand writing if and for statements. I like the look of ipythonblocks, because it is simple, concise, and clear. I haven’t tried actually running it yet, but I expect that using this viz tool would facilitate student learning greatly. I doubt that it draws the squares one at a time (showing the iterative process, almost like Turtle Graphics), but I would guess that functionality would also help with student understanding. Below is a picture of a repetition and selection in Python, visualized with ipythonblocks.
Teachers could then ask their students to do tasks like “turn all blocks in the second column yellow” or “change all red blocks to blue and all green blocks to purple.”