This is what I use for giving simple, quick-to-produce presentations. Rather than fighting with keynote or powerpoint for hours, i can whip up a presentation in minutes using markdown.
Check out a demo here. Use the arrow keys on your keyboard to switch between slides.
You need to install this on a web server, otherwise it won't be able to open the presentation.md file via AJAX. So, if you're on a mac, copy it to your ~/Sites/MyPresentation folder. Then open your web browser to http://localhost/~myusername/MyPresentation/Presenter.html.
The presentation.md file is where you put your presentation. All you need to do to separate slides is a paragraph with an exclamation mark, eg:
This is a slide Blah blah blah ! This is another slide Yada yada yada