This plugin is now shipped with xfce4-panel >= 4.4.0.
Written by Andre Lerche, this plugin allows you to hide and unhide unsticky windows on the current workspace with one click. You can unhide all windows with a left-click, or just unhide the last window group with a middle click.
Just add it to your panel. Clic once to get the desktop, then clic once again to return to your application. Tha's all!
Is now shipped with the panel, so there is nothing more to download than the panel.