Windows/Mac/Linux (all systems): Impressive doesn't make slideshows, it just makes them look better. The free app's smooth transitions, zoom-out grid views, and screen-dimming highlights can be used with just about any slideshow. The Python-based app (read: goes pretty much anywhere) takes in PDF files exported from most any slideshow maker—PowerPoint, Impress, Keynote, and many more—and loads either from the command line or by simply dragging your PDF onto it. The app launches, your slides load, and it cycles through a series of transitions that aren't too hard on the eyes (or indicative of your My First PowerPoint skills). Hitting Tab gives you a slick grid view you can zoom to any slide from, and you can dim slides and highlight key points with a mouse, or use a roaming spotlight with the Enter key. I had some trouble getting the transitions to record in Windows 7, so I reverted to the ScreenToaster web recorder I'd tried out before and my wife's laptop. The transition effects are still missing, and there's a bit of frame drop, but the other features are visible in this demonstration (using an old PPT I found lying around the net):
Impressive is a free download, works wherever the Python scripting language does.