Most elegant UI for categorizing items?

Question

I have a collection of items that the user needs to group/categorize in several ways. For the sake of an example, let\'s say it\'s a collection of cars and the user wants to cat

Answer 1

I may be misunderstanding you question, but isn't that very similar if not exactly what tags are for (as in stack overflow and gmail). Or are you looking for something more specific than that?