SlideShare is a fantastic platform that allows you to upload PowerPoint presentations, Word docs, Adobe PDF files and even Keynote stuff (with a little converting to PDF!) I've uploaded a couple of presentations so far and seen an amazing amount of views. My most recent SlideShare upload about podcast intros - http://www.slideshare.net/musicradiocreative/podcast-intros-pro-podcast-intros-attract-more-listeners - had 25 views in the first hour! Not only can you share presentations but you can record a Slidecast. Which allows you to record audio to match with your presentation slides!

SlideShare : Slidecasts: http://help.slideshare.com/forums/326091-Slidecasts