The folks at SlideShare are trying to build an equivalent to YouTube for presentations rather than movies.
The site is still invite-only (got the approval in minutes!), but you can see what the uploaded slides look like at TechCrunch, or you can have a look at the slides I presented at the OpenEd conference.
The main problem with SlideShare at this moment, it seems to me, is that it doesn’t enable you to repurpose existing presentations: that is the focus of our work on embedding repository, share and reuse support in powerpoint. Seems like there may be some complementarity here? (Is that even an English word?)