As you can see I’ve added in a category listing to the sidebar.  A couple of points.  EE allows sub-categories.  Content, Design and Navigation are, in fact, sub-categories of Blogging.  EE does allow you to display them in such a way that these relationships become clear.  Unfortunately, everything on the sidebar is right-aligned and doing it EE’s way would look silly.  Not quite sure how to solve that one.  The other point is that when you navigate to the page for a parent category you will get all the posts for the sub-categories.  This is not a problem just yet but it could easily become one if we end up with one category containing lots of posts.