dhtmlxTree is an Ajax-based JavaScript tree menu that allows to create full featured navigation systems and organize large amount of data into hierarchical order.
Dynamical loading of items from XML stream gives a possibility to create trees with unlimited number of nodes.

dhtmlxTree can load its content dynamically from server using Ajax or directly from on-page XML data-island. If the tree has hundreds of nodes, subnodes can be asynchronously loaded on client-side and then displayed by portions on demand. It's possible due to distributed parsing functionality implemented in dhtmlxTree.

Powerful client-side API allows complete customization of a tree view with JavaScript. dhtmlxTree allows users to edit text of the tree nodes right in a web browser. All the changes performed over the tree (text editing, nodes reorganizing) can be automatically saved in backend database on server side.

Extended drag-n-drop functionality greatly simplifies tree usage. Drop-as-sibling behavior makes it easy to reorder nodes in a tree view. The tree control supports drag-and-drop not only within one tree but between trees, even if they are located in different frames or iframes. You also can select and drag multiple tree items.

This DHTML tree has checkbox system in order to use this component as a navigation bar with selection lists. Besides usual two-state checkboxes (checked item, unchecked item), there is a possibility to use three state checkboxes with "partly-checked" value (some sub-items are checked).

dhtmlxTree is distributed under both GPL and commercial license. Source code is available.