AuthorStream is a great site to upload/share powerpoint documents. In a way it's similar to slideshare w/ more options. Once a user uploads their slideshow, they can either embed them into their site, make them available for download, link to it directly, or even create their own "room" (think YouTube) to "house" all of their presentations.
This really is a neat site to take your powerpoint presentation to the next level.
Give AuthorStream a try by clicking here.
I highly recommend checking out Shelly Terrell's wonderful blog for more excellent ideas/resources for your classroom!!!