DScreen

This application permits you to run a screensaver as a desktop picture.
You can choose one of your screensavers and view it under your desktop.
DScreen has its personal preferences, so you can have one screensaver for it and the standard screensaver for system default.
However you can select "Default" to use the some screensaver of the System Preferences.
DScreen doesn't invent nothing, it only use a secret feature of ScreenSaverEngine (the application that play the screensavers) without passing by the Terminal and so is very simple to use.