In Delicious I liked very much
collections - a kind of special tags for tagging other tags. Thanks to that one tag could belong to many collections. This worked very well - great trade-off between mess of total hierarchy (ontology) and complete lack of it. One of discussions about collections on Zotero forum.
People working on Folksonomies discuss that hierarchies might be overrated and tag cloud might be enough or machine learning in order to perform some clustering.
What I like, are non conventional approaches, as I thing the solution should emerge from mixing many of current approaches in order to make something flexible.
Here is discussed concept of ordered tags, where flat tagging is extended with information kept in their order. On the one hand it give much more power to traditional tagging, on the other is not in conflict with standard concept at all.
Another innovative approach worth mentioning is Kleenk system for collaborative organizing research papers. It's uniqueness is organizing papers through relations between them, as it happens in our minds. Of course relations bring semantic information about meaning. We do not have collection of tags and tag things. We have collection of concepts (which might be things, tags, abstract, matter... whatever) and we connect those with relations.
Kleenk.com is approach of providing way of doing so in visual collaborative way. I would love to see more tools like this for other stuff (we sites, music... everything), maybe combined with previously mentioned approaches.
As summary I might try to list those in list from flat to hierarchical structure:
- Flat Tagging
- Ordered Tagging (Flat tagging extended by ordering)
- Collections of Tags
- Tags with Inheritance
- Hierarchical Tagging
- Tags with multiple inheritance
- Graph of Tags - Relations
- Graph of Tags - with relations consisting meta-information (like semantic)
Where do you put which - it's up to you. I believe there must be works on comparing those or showing some properties (maybe formal homomorphisms, isomorphisms). Those are just systems of organizing information.