wpquestionI have looked around and haven't seen any plugin that can allow one-to-many or many-to-many category structure. The hierarchy is based on a parent/child relationship.
Best described with an example.
Let's say you want to do a recipe site. You create categories for:
Ethnic Recipes -> Italian Recipes -> Italian Dessert Recipes
But you also want the category of:
Desserts -> Italian Dessert Recipes
So you have to choose a primary category for the post since you don't want to duplicate the post or the category.
What you need is a pseudo category. The first location is really where Italian Dessert Recipes category really exists, but the link to it (a pseudo category) should be able to show up under Desserts.
There are thousands of examples of where this would be applicable. Book sites, Directories, or any category structure that would have multiple ways to get to the same post.
Is there a plugin to do this? Are there plans to integrate this into WP?
