Simon makes a note of the rather interesting-looking LUFS-Python, a tool for writing filesystems in Python. He says:
bq. At first glance, this is a bit of a gimmick – why would you want to write your own filesystem in the first place?
...and then goes on to list pretty much all the ideas I immediately thought of as to why this would be good :) A filesystem that automatically ran Tidy on HTML files, for example. I’ve toyed with the idea of this sort of thing before and been disappointed that there’s no easy way to do it under Linux—the Hurd has stuff that can do this, but Linux doesn’t. It’s also not in userspace—you need the kernel LUFS framework in your kernel to do this—but it’s cool anyway. One to think about in more detail.