Powerful document trees?

I'm looking for examples of navigating through 2-2,000 documents that are more elegant + powerful than the stereotypical document tree… but also has research backing it would be nice.

Currently the documents are arranged by

- Category
  - Sub Category
    - Documents (1-100 pagers)
      - Date

I've seen elastic lists which is interesting. http://well-formed-data.net/experiments/elastic_lists/

Anything else come to mind?

Thanks,
E