Deckset for Mac
Deckset is a Mac app for making slide presentations with Markdown. Compose your thoughts in your favorite text editor, send them to Deckset, and it uses one of several themes to give your words a little flair.
Deckset recognizes Markdown formatting for headers, code (with automatic syntax highlighting!), blockquotes, and more. You can also add photos and videos to your slides. It may not replace Keynote or PowerPoint in terms of sheer power, but it's plenty enough for most needs.
Get Deckset for $20 on the Mac App Store. Also, be sure to check out the MacStories review.