Check out the demo at http://lab.hakim.se/reveal-js/
It structures slides "horizontally" like in a classic presentation, but from each slide, the user can "drill down" vertically into children slides. Slides can also link to other slides.
* WYSIWYG editor - http://rvl.io
* cross-browser support, including IE8
* easily themable using CSS; five themes provided
* cool 3D transitions (4 effects)
* keyboard navigation
* progress bar
* overview of all slides
* zoom into any slide with Alt+Click
* can run offline or online
* PDF export
* gesture support (swipe, pinch)
* Markdown support
* syntax highlight for code
* "pause mode"
* named internal links from one slide to another
* does not use Flash
As of August 2012, there is no GUI available for creating presentations. However, a basic knowledge of HTML will be sufficient.