One of the newest widgets in our product is the jqxTree. It represents a highly configurable jQuery Tree widget that displays hierarchical data, such as a table of contents in a tree structure. jqxTree can be generated from lists and standard anchor tags, which are properly recognized by search engines. As a result, all content accessible through this widget will be automatically indexed and ranked with no extra effort required from the web developer. The Tree widget comes with built-in keyboard navigation, animated expand and collapse, customizable toggle mode, flexible API and professional themes. You can also display a checkbox next to each tree item and check or uncheck all sub-items whenever the parent tree item is checked or unchecked respectively. An indeterminate state of the checkbox indicates that some child objects in the collection are checked while others are not.
See the jqxTree in action:
jquery widgets demo
See how to add the jqxTree to your pages:
jquery widgets documentation 
By far this is the best i’ve seen. It would be great if data tree populated by json / server page could have a refresh method to update the tree whenever data on server side is changed.