I'm currently thinking about adding a quite comprehensive section to a website: the section I'm talking about starts at level 3 of the overall website and itself has another four structural levels. Massive!
I cannot use or touch the horizontal nav of the website which is used for the first two levels of the website. I was looking for an alternative to left-hand navigation which still offers high flexibility for the user.
So, my idea was to use a sort of breadcrumb navigation - through each item in the breadcrumb would be a drop-down menu. Each selection in a drop list would populate the next dropdown with the subpages of the selected page. As well a selection would open the corresponding page.
So, the user could use the drop-downs to move forward and backward - but as well to switch categories or sub-items.