Photoshop is worth owning if you're adding more than just simple photo effects. But for most of us, there's PhotoFiltre, the best free photo effects software.
Best Photo Effects Software
Here is the screenshot of PhotoFiltre. As you could see, it has a Photoshop style interface but makes it easier to apply photo effects to your images. PhotoFiltre is more practical than Photoshop by including useful tools like batch converter, icon exporter, image explorer, etc.
Photo Effect Slideshow Software for Windows
DVD Slideshow Builder contains general photo effects for slideshow photos.