I just tested 280 Slides and I am kind of impressed how comfortably you can create simple screen presentations without having to use an installed tool like OOo Impress or MS Powerpoint. It's got a nice GUI like Apple Keynote and offers basic tools like inserting pictures or movie clips.
But I also have to admit, that I did not try Google apps or similar services yet.
At 280 Slides, you can download your presentation in the MS Office 2007 format
pptx. So you have got at least something in case the services goes down.
Note: this blog entry was originally authored using Serendipity and converted to Org-mode format for publicvoit via a dumb script. This may result in bad format or even lost content. Please write a comment if you want to get in touch with me so that I can try to fix things.