slideSaver Maker is a free and easy to use software that allows you to create your own slide show screen savers, like the "Forest" and "Beach" screen savers which come with Mac OS X.

When you start slideSaver Maker, it should ask you what you want to call your screen saver.  Name it whatever you want, but be sure to save it in /Users//Library/Screen Savers/ or else it won’t show up in your Screen Saver settings panel. 

You can also put it in /Library/Screen Savers/ to make it available to all users on your computer.

After you give your new screen saver a name, a window with some instructions will appear.  All you need to do is click the button to open the screen saver’s Resources folder in the Finder and then fill the folder with pictures.