Treeview Plugin based on Categories and Pages/Posts

Is anyone aware of a plugin that would render pages or posts in a treeview-like structure (based on category), such that the items in the treeview would be navigable via mouse (within their hierarchy)?

An example might be pages or posts for 200 cities in the US, and each city page/post is associated to a state (via the category hierarchy). What I’d love would be an initial listing of the 50 states in a treeview (either on a page, or in the sidebar), and you could expand a given state via a + sign, and see the cities expand beneath (or the page/post for each one), and can then click on them, and work on them.

Make sense? Any ideas?

Thanks!

John