Thanks, Macworld's Mac OS X Hints:

It turns out that the screen saver is just an application, so you can put an alias to it in an easy-to-access location, such as your dock, or the Finder’s sidebar or toolbar. Just navigate to System -> Library -> Frameworks -> Screensaver.framework -> Versions -> A -> Resources, and then drag onto your dock, sidebar, or toolbar. Now when you want the screensaver to activate, just click the convenient icon.