Apple - Is it possible to show a screensaver on a secondary monitor while using the primary monitor?

You can set your screensaver to be your desktop background (link to source).

To do so, run this in the Terminal:

/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background

This works on both monitors (tested just now in Lion), so as long as you don't put any windows on the other monitor, it'll just show the screensaver. (Unfortunately, the screensaver will also be behind all of your windows on the screen you're using. I hope you really like your screensaver.)

The easiest solution is the widget Wallify. It is free, with a "Pro" version available, but the free version basically does the same thing that CajunLuke suggested, but without having to use the Terminal.

You can select most of your screen savers directly from the menu on the widget, but specific photo slide-show style screensavers don't appear as an option. To use these kinds of screensavers, you can choose them using the System Preferences > Desktop & Screen Saver, then on Wallify just select "Default", and it will use whatever you chose in System Preferences.