The Magic of Base Ten Blocks: Develop Conceptual Understanding in Guided Math Groups
This is a great site that shows you how to use base ten blocks to develop conceptual understanding of the operations. I would do all of these activities with the students in small guided math groups with real base ten blocks. I would then hold small guided math sessions at the computer so I could use this site to teach at the pictorial level. Finally, I would have the students work with this site during center time.