SilkSlides is a service I have been hearing buzz about that allows you to upload a set of slides (PowerPoint or PDF) and have the ability to discuss the content of each slide. It takes 20 seconds to upload the presentation and within seconds, a URL is emailed to you so that you can link it to your website. Once the students access that web address, they are able to view, discuss and download the presentation. When commenting, it does require that they post their name and email address (even though it only includes name on the comment post).
One thing you need to be aware of is that you need to save your presentation in .ppt or .pdf format in order to be able to upload it. They are working diligently on making more formats available however. If you would like to see how it looks for students and experiment with commenting, please feel free to do so on this test presentation.