Have a look at myCourses and siteNavigation Blocks. Code could be customized to show in a ComboList or in the central area on the FrontPage or simply anywhere you like, even in a separate "all courses" List of your own.
Have a look also on my TestServer, where I use another neat category/courses Menu which opens horizontally (from Kevin Treussier). That's why I can not use it, because our large scale site with more than 4'000 courses deeply nested in more than 1'000 categories would scroll infinitely to the right.
So I created the siteNavigation out of the myCourses Block which both show hierarchical nested folders.
Actually I am recoding to show both in a popUp-window to have more space than in the 210 Pixel wide Blocks.