Publicly edited categorization

I want users of my site to be able to classify their projects by categories.

Both extremes:

• a fixed set of categories

and

• a set of categories arbitrarily edited by users

are not quite good for obvious reasons (fixed set is well too fixed, arbitrarily edited list is going to become well arbitrary).

Is there any known way to allow the users to edit the lists of categories and subcategories, but disallow them to create chaos or for example creating a new too thin categories tailored to a single project, create duplicate categories, etc.?