I created a set of easy to use linked list controls that update on the client without a postback.  All of the data for the lists are downloaded when the page loads.  The only requirements to get the full feature set is a DataSet with DataRelations as the DataSource, and setting a ParentListID property in the controls.  The ParentListID is very easy to set.  The property dialog lists all compatible controls on the page.  Just pick the one that will hold the parent data from the DataSet. 

There are no extra, hidden controls to configure.  All the code is contained in the list controls.  The controls automatically find the DataRelations and set up the relationships between the lists based on the ParentListID property.

The assembly contains two controls, a RelationalDropDownList and a RelationalListBox.  They can be mixed and matched in the same grouping.  A normal DropDownList or ListBox can even serve as the top level control.

Check out the demo.  I have it in a Beta status for now until i get some feedback although it is stable as far as I can tell.  Let me know what you think if you use it. 

There is a download link on the demo page and a link to online documentation.  The documentation is also included in compiled form in the download as well as a sample site.

Relational ListControls demo and download