You may want to try the Screenlet FolderView. It will not give you the roll-over effect, but can help you categories elements on desktop.
To install Screenlets, run the following in a terminal
sudo add-apt-repository ppa:screenlets/ppa
sudo apt-get update
sudo apt-get install screenlets
The FolderView screenlet will show you the content of a folder in a "fence" -- hence, for each Folderview, you have to have the stuff you want shown in it in a separate folder (located just somewhere, not necessarily on your desktop). If you want to "clean up", you can just close a given Folderview.
Here you see the main Screenlets Manager, the FolderView settings where the portrayed folder is chosen and the screenlet it self on the right. There are a lot of customization options with respect to the look of each FolderView screenlet.