Finally, tagging for files! I stumbled upon this on Stumbleupon and so far this is a neat addition to the traditional file hierarchy system... only that I have some suggestions:
It would be great if when I drag a file on the program, it would try to guess suitable tags from those that I have already added. From my perspective this would be rather easy add. Fuzzy string matching would be needed but this too of course is elementary stuff. I can even supply some code, because I want this feature so badly.

The rest of my suggestions are normal user interface stuff: a list of recently used tags, no need to confirm every addition and so on.
Good luck with your project!