Express Points is a free Windows and Mac tool for designing and showing slideshow presentations.
The program works much like a (very) simplified PowerPoint. 24 master slide templates help you build your presentation; you can customise each slide with text, bullet points, lists, shapes, images and audio files; and objects can be moved, resized or rotated to suit your needs.
A vertical timeline shows all your slides in sequence. You're able to rearrange and reorganise them as required (or add one of the 34 transitions), and the slideshow can be replayed when required to your audience.
There are plenty of limitations here, of course. Express Points can import PowerPoint PPTX files, for example, but the program will just ignore everything it doesn't support. These can be fairly basic elements, too, like slide backgrounds. The PPTX support may save you time, but don't expect too much.
It's not easy to collaborate, either, as you can't export your slideshow to PPTX or any other kind of easily editable format. It can be saved as a video (in all the main formats) or PDF, though, so you can at least show others what you're doing.
Still, Express Points does provide a solid set of basic presentation tools, and even partial PPTX import is better than nothing at all. If you're looking for a lightweight, simple way to create local presentations then we'd give it a try.
It's not exactly PowerPoint, but Express Points packs in more functionality than we'd expect in a free tool. Worth a look.