Do you intend capturing the talk to deliver as on-demand after the event?
You could try Windows Movie Maker software programs for presentation...
The open source alternative to MS Office is LibreOffice. It supports MS Office file formats and can export PPT presentations to Flash and accessible HTML pages with text alternatives, etc.
I can understand your wish not to perpetuate the "death by PowerPoint" talks that so many speakers give. I recommend looking at ted.com analytically and critically to get some ideas. I particularly liked this one by Hans Rosling: http://www.ted.com/talks/hans_rosling_on_global_population_growth.html
I wrote an article about open source software here: http://blog.matbury.com/2011/10/19/open-source-software-for-elearning/