I've been looking for a wiki for an internal knowledge repository where users can post solutions but be able to place them in a category so they're not just thousands of stand alone pages with links.

Searching through a large wiki for information on a specific subject can be a nightmare without some sort of categorization.

I noticed an aweful lot of people looking for a tree type organizaitonal structure and a feature that allows users to place their solution article in a category.

MSDN does it, and it works great!  You can drill down to the information you're looking for or search for it.

if anyone develops one for the current wiki I'll let me know!

Any updates on this? I'd like to get one for my site if possible!

Currently there is no categorization/tree structure to the Wiki. 

Perhaps it's just me, but a Wiki is a freeform application. You can create your own categorization structure by creating links and top level pages. Is that not sufficient for people?  

I've done that with the DNNDocs website http://www.dotnetnukedocs.com/