dTree
I was looking this morning for some solutions to build a tree navigation (client side). I found this Javascript API easy to implement and surely ready for an Ajax modification.
dTree is a free JavaScript tree menu.
There are a lot of tree generating scripts just like this one, and many of them are great. So why use dTree?
dTree is very simple to set up and use. You don't have to worry about adding pages to the tree in a specific order, which makes it ideal for generating a tree from a database.