The Treeview skin has finally made it out of alpha and is available for public use as a beta. The skin adds a dynamic and configurable javascript-based treeview component in place of Monobook's navigation sidebar, supporting load-on-demand when expanding nodes. It also adds a couple of other niceties that you can see here: http://clc-wiki.net The skin can be downloaded from here: http://members.dodo.com.au/~netocrat/software and feedback can be viewed/added here: http://clc-wiki.net/wiki/Planning:Treeview_skin:Feedback
The skin is best suited to small-medium sized sites with relatively low edit rates and with a MediaWiki cache manager enabled.