TagStore is a PIM research prototype that provides a way to tag files and navigate to them using a tag generated hierarchy (I already mentioned it on my list) in addition to a regular hierarchy. It is not the first such system as many similar were developed before (TagFS, Semantic File System, etc.), but you can download it, it works on Windows, Linux and OS X and it is open sourced. But beware, it is a research piece of software.
I’m not particularly good at tagging things (check how I tagged my blog posts a while ago and how I do it now) and I’m also not a big fan of such systems, but you might like it.