explorer like tree view in rails ?

Hello,

I am looking for recommendations on a plugin or javascript lib that support building an explorer type tree view. I am somewhat afraid that the more extensive things available are fairly complex, and we want to use something free ..

I am using acts_as_tree and creating entries from file directories and then displaying some of that info on a web portal with special custom functionality that we need for our app.

I need to be able to build leaves of the tree on the fly with ajax as well since creating the whole tree may take too long etc.

Thanks ..