My new friend Angie mentioned The Workbox System at our homeschooling group Tuesday night. It sounded great! I did not research the whole system. Instead, I just got a few boxes and separated Aiden's schoolwork out between them. When he has gone through all the boxes he knows he is done with school. It lets him know where he is at and how much more there is to do. Eventually I want to add chores and other things to their boxes, but for now I am starting with the basics. I went with the modular latch boxes that stack together (from Wal-mart).
I'm also posting a picture of some tall skinny file boxes that might work well for some (from Office Max).