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 DinnerStep 2:
Chinese Beef Mongolian Beefor
Chinese Dinner Mongolian BeefDrill-down B:
Step 1:
Beef American ChineseStep 2:
Beef American Chicken Fried Steakor
Beef Chinese Mongolian BeefPaper (from the Haystack project): "User Interfaces for Supporting Multiple Categorization" try: Applications that use this technique:
Haystack - http://haystack.lcs.mit.edu/
Bibliographix - http://www.bibliographix.com/
See Also: LimitsOfHierarchies