Making nested tree navigation?

I had a question the other day about making a "breadcrumb" trail which I eventually completed, although not exactly the way I wanted to.

Anyways, now I want to create a "directory structure navigation". Each category has a parent id, so how could I build it to be like:

-Main cat
----Subcat
----Subcat
--------Sub-subcat
----Another subcat
-Another main cat
----Subcat

I *could* just get each category row, using the parent id and then store everything into arrays and then build it like that, but there is surely an easier solution to accomplish this.

These are more common too, so hopefully someone can help me with this one!