Wiki sites?

As part of my assessment this semester, I'm making a wiki. Basically, the wiki will be used to demonstrate what I'll be talking about, which is Web 2.0 and libraries. So I've been spending
some time today looking at wiki sites.  

So far I've looked at two, PBWiki and WikiSpaces.  PBWiki was slightly frustrating in that it
doesn't appear to have a way of creating a navigation structure.  There is the sidebar section, which from what I can tell, you can edit to have as a navigation tree, but there is nothing that indicates that.  Also, the help doesn't have anything on navigation, and the sidebar page in the FAQ is empty!

WikiSpaces I like a bit more, however it does have ads that take up a quite a bit of space on
the side. I've used WikiSpaces before, so I know how it works, but I would preferably like something with not as many ads.

I do have wiki software on my personal hosting, but I'm going to try to stick with freely 
available web based applications for this.  My next target will be MediaWiki.