You can always use tags and then build a taxonomy later based on the tags.

A project I developed (an inhouse used equipment sale/trading site - which eventually got canned before release as the business decided to get out of that part of the business), but we used tags and a tag cloud to allow flexibility to the users and administrators of the system and then used the tags that emerged to make some hierarchies amongst some of the tags.

The tags were great for maintenance, browsing and search, and then identifying certain popular emergent tagging behaviors and building hierarchies later allowed us to make very malleable hierarchies (since the hierachy was simply a many-to-many tag relationship, and had nothing to do with the items being tagged.)