Unique pages with unique menus

I’m building a site which will eventually have about 6000 pages and in a perfect world I would be able to break down these 6000 pages into about 15 top categories and then some of these categories into anything between 8 and 30 lower categories and so on. A perfect way to do this would be to have pages that have different unique menus in the sidebar.What would be the easiest way to do this?