SlideShare is the world's largest community for sharing presentations on the web. You can

• upload & share your presentations (ppt, pptx, odp, pdf, keynote)
• embed into blogs & websites
• share on LinkedIn & Facebook
• create a webinar by linking slides with audio
• embed Youtube videos into them
• access SlideShare on your mobile

And now SlideShare also supports all documents formats (doc, docx, odt, Mac iWork pages). And it's completely free.