This plugin is an entire gwt rewritte of the JQuery-ui droppable plugin
The Droppable plugin enables any DOM element and GWT widget to be droppable or in other words a target for draggable elements.
More information available at : http://code.google.com/p/gwtquery-plugins/wiki/DroppablePluginGettingStarted
Examples are available here
Original author of this plugin: Julien Dramaix