This problem is pretty much solved by Adobe Lightroom, although not at filesystem level, purely within the application itself. You tag photos with keywords, whi

This problem is pretty much solved by Adobe Lightroom, although not at filesystem level, purely within the application itself. You tag photos with keywords, which can be hierarchical. Let's say you take a photo of a frog in Florida at night. You apply the keywords "frog, Florida, night". Over time you photograph all kinds of other creatures as well as across several locations, so this list of individual keywords will grow. The cool thing is that you can now group the keywords themselves. You create a new keyword "USA", and then drag "Florida" in it. You create a new keyword "Amphibians" and drag "Frogs" in it. And then create "Animals" and drag "Amphibians" in it. The powerful thing is that you now have the power of flat keywords as well as hierarchical browsing across multiple hierarchies.