Apple's iPhoto app has some very basic slideshow features built in. Sure, you can apply the Ken Burns effect all day long, and even save your slideshows as screen savers. But if you are bored by iPhoto's simple dissolves, pans and wipes, it's time to check out ScatterShow (US$29.99) from Smith Micro.
The OS X app (which can be purchased as a download or in a physical box) allows the creation of some really unique arrangements and animations of slides, all of which can be set to music. The app includes a number of pre-set templates, or you can choose to start from scratch. Once you select a background from those provided by Smith Micro or have added your own, you're practically done making your slideshow. Images for the slideshow can be selected individually, or you can add a complete folder.
Each photo can be converted to black-and-white or retouched with a variety of filters. The app provides a simple way to change the position of each image, and when the slides are finally animated, the app moves from image to image in a visually interesting presentation. Music is supplied from your iTunes library, or any music file can just be dragged and dropped into the app.