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 Mac
Now, if you want to make a slideshow with good-looking photo effects, there is no need to find another photo effects software. You could always edit your photos with built-in photo filters. Taking Fantashow for Mac for example, it comes with more than 15 photo effects to enhance your photos in one click, including Black and White, Sepia, Negative, Cartoon, Vignette, Hot Wave, and so on.
Furthermore, the supplied gorgeous styles actually add more effects to your photos in slideshow, such as these:
Photo Effect Slideshow Software for Windows
DVD Slideshow Builder is the Windows equivalent version to Fantashow, which also contains general photo effects for slideshow photos. What should be mention is that it has more styles than Fantashow to create movies with stylish photo effects.