Nice Tree Navigation System

Of late, I had to develop a navigation system for a couple of applications and based on the analysis and study, we thought of going for a Tree-based navigation system, which would be quite user-friendly for the user. Whilst searching and analysing the various navigation systems, I found a couple of sites, which I would like to share the URLs with the web development fraternity, who may also be in search of site navigation systems:

http://www.treeview.net/
http://www.blueshoes.org/en/javascript/tree/ (A lot of trees with even checkboxes etc, besides the nodes)