While you can use a variety of tools to create basic slideshows, Keynote may be the most powerful tool for quickly creating interesting photo slideshows. You can instantly create a slideshow by dragging and dropping photos from the Finder into the thumbnail list in Keynote. You can then add transitions, text, shapes and other objects to slides as needed. You can also interject other types of slides. There are many export options as well, including video and HTML for your web site.