This free app seems to be a cross between Keynote, Haiku Deck, and Polleverywhere. Some interesting features including templates and polling capabilities.
This is an interesting classroom management concept for 1:1 situations when teachers want to control the presented content. Not sure about the ability for students to interact with content, but they could take screencaptures and add to their notes easily. Lots of potential.
The lite, free, version of this web service lets you create presentations and import them from either PowerPoint or Google Docs. There is also an iPad/iPhone app.