Mac: PowerPoint and Keynote have been the main presentation tools for most people for a long while, but if you're looking for a simpler alternative, Deckset makes things a bit simpler by building presentations based on Markdown files.
Instead of using a visual format to design a presentation, Deckset lets you just type it out in Markdown using a regular text editor. When you're done, open that file in Deckset and it's automatically turned into a presentation.
You can pick from a number of themes, colour palettes, and add images to spice things up. That's it. It's incredibly simple, but that's also the point. Deckset isn't really a replacement for something like PowerPoint. Instead, it's a tool for people who don't really want to mess around with visuals and would prefer to just present their ideas in a simple way.