Slideshare is something I have recently discovered and I'm saddened that I didn't discover it any earlier.
In brief, Slideshare is about sharing your powerpoint presentations, PDFs and videos where it is viewable to the public. People can link to your presentation and download it as well.
It definately is an effective marketing stragety because many users are jumping on board daily. It will become the new Youtube for documents. What an innovative idea!