The gwtquery draggable and droppable plugin lets you make any DOM element draggable and/or droppable. In addition, it offers an API which lets you add drag-and-drop support to your GWT widgets, including also the new data presentation widgets (aka cell widgets).
This bundle contains everything you need to use gwtquery and its draggable and dropable plugin.
More information available at : http://code.google.com/p/gwtquery-plugins/wiki/DragAndDropPluginForGWTDeveloppers
Examples are available here