Presentation
If you're looking for alternatives to Powerpoint or basically want to put up an online presentation and in turn reducing the issue of the viewers needing a particular software then there are a few sites worth looking at.
If you're looking at a close to standard Powerpoint based presentation then have a look at 280 Slides. It allows you to import your powerpoint presentations and if you're going to create them from scratch then it offers just about everything Powerpoint does.
But, if you're looking into something different then Prezi offers something different with its zoom in and out functionality, Prezi is being taken up by teachers due to it's ease of use and it's zoom function. It's not for high-end presentations where you're looking at putting in menus and making it interactive but more as a strict presentation tool.
Finally if you're looking at an interactive soft of presentation, then wix is the way to go. I've mentioned Wix earlier of course and it's still perhaps the best online application to create flash websites/presentations in, it's easy to use and you can create some pretty amazing sites/presentations.