This is a UI pattern being explored by the Haystack project at MIT (link below).  Essentially, you take an item (like a bookmark), put it in as many categories as appropriate, and you browse down through a dynamic hierarchy that at each step represents the intersection of all categories you've drilled down through.

Example:

A: Web page that contains a recipe for Mongolian Beef goes in categories: Chinese, beef recipes, dinner recipes
 
B: Web page that contains a recipe for Chicken Fried Steak and Eggs goes in categories: American, beef recipes, breakfast recipes

Drill-down A:

Step 1:

 Chinese
  Beef
  Dinner

Step 2:

 Chinese
  Beef
   Mongolian Beef

or

 Chinese
  Dinner
    Mongolian Beef

Drill-down B:

Step 1:

 Beef
  American
  Chinese

Step 2:

 Beef
  American
   Chicken Fried Steak

or

 Beef
  Chinese
   Mongolian Beef

Paper (from the Haystack project): "User Interfaces for Supporting Multiple Categorization" 
*** http://haystack.lcs.mit.edu/papers/interact2003-multicat.pdf ( BrokenLink )
try:
*** http://www.academia.edu/2869086/User_interfaces_for_supporting_multiple_categorization


Applications that use this technique:

Haystack - http://haystack.lcs.mit.edu/

Bibliographix - http://www.bibliographix.com/

-SteveConover

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See Also: LimitsOfHierarchies
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CategoryOrganization