One of the open source packages we have used for custom development work in the past is OSCommerce. Being a visual learner it helps me to be able to "see" the schema. Here's a graphic I made with the help of DB Designer 4. I took a stab at filling in some of the relations, though they aren't complete nor are they completely accurate. Also ignore the tables in the upper left starting with fg, those are some views we wrote that got included in the schema. DB Designer 4 seems to no longer be developed or supported. MySQL workbench provides similar functionality, but the free version doesn't let you print or save the schema as an image. Unfortunately db designer 4 doesn't export at super high resolution, so if you want higher resolution I suggest downloading mysql workbench and giving it a go.
