I’ve posted in the past about web applications to create online slideshows, and have many links to these sites under Student Slide-Shows on my Examples of Student Work page.
I just added a new one called PreZentit. This is a little different from the others because it’s supposed to allow multiple people to work on the same PowerPoint-like presentation at the same time. This would obviously come in handy if students were working on a group project.
The Mashable blog also recently wrote about over thirty different online slideshow presentation tools.