ArtSaverApp provides a "slide show" or "picture flow" for large image collections you have stored in a folder (or several folders) on your disk.
With ArtSaverApp, you get more control than with Apple's built-in screensaver: at runtime, you can review previous images (using arrow keys) and a few other controls; in the settings, you can control exactly which images should be displayed, the duration, how it switches from one to the next, which information to display with the images, and more.