I just finished piloting 'silk slides'. Silk slides takes a slide show and turns it into a more collaborative experience online. I usually use "slideshare" to upload powerpoints for embedding like below, but taking in the information is a little "passive learning".
With Silk Slides, you upload a slideshow and it gives you a URL. The students don't even need a username and account. They go to the URL that is spit out after an upload. You can pepper a slideshow with questions that students can comment on with each other and read each others comments.
The sample was a discussion prompt that I had at the end of my lesson on comparing plant and animal cells.
The only real downside I see with silk slides are that there are some formatting issues that don't align perfectly after an upload. Other than that, it's worth a whirl.