Ajaxified tree comes with the following features
1.) Containing the ajax links for the tree items using link_to_remote
2.) Highlighting the selected node of the tree and other custom effects.
3.) Two way syncronization between the tree nodes and the currently displayed item on the web-page.
4.) Supports the tree architecture provided by the acts_as_tree.
5.) Clear and self traversing [...]

We have been doing a lot of ajax development recently.
We have been developing ajax applications with xajax, the ajax library in php. Also we have been using ruby on rails, with the scriptaculous and prototype library.