subfolders in Calibre?

I understand that Calibre does much of its organization via tags. That said, I'd rather not use tags; can I somehow – i.e., via a plugin or other functionality – simply make more subfolders in the sidebar and sort my books that way? This way I could keep a single library and just break them up by whatever kind of category I'd like, ala other apps that have the same general look/feel as Calibre (i.e., itunes, etc).

Is this even possible? I can't seem to be able to figure it out, although it looks like it should be.