Dropbox-treeview is a project mainly written in JAVASCRIPT and RUBY, it's free.
Combining use of YUI 2.0 TreeView widget and Dropbox API
This gem is supposed to provide an interface to a Dropbox user's Public/ folder by way of the YUI2 TreeView widget.
So as not to abuse the Dropbox API, the entire directory structure is not loaded at once, but incrementally as the user browses the tree.