2 Tier Dropdown Menu

Hello,

I need to make a navigation menu that drops down twice to organize my products better and also spread out my products a bit more so that the page doesn't take so long to load. So for example;

Inventory
--> Category
              --> Tag

Pretty much what I need is to be able to hover over the "Inventory" title along the main navigation bar. When I hover I can move my mouse down and hover over the "Category/s" then this will bring out another menu for the "Tags" which I can then click on and bring me to the product pages. If someone could help me with a code or some other way of doing this that would be great! This would really help me to have this feature. 

Thank you!