Slideshare

SlideShare is a community for sharing presentations. Individuals or organizations can upload and share PowerPoint, PDF, or OpenOffice presentations.
Users can tag presentation, and download or embed them into their own websites or blogs. Users can also share their documents privately.
SlideShare lets its users to join groups to connect with SlideShare members who share similar interests. Business presentations make the most of the content. SlideShare also has a facebook application for uploading documents

Techcrunch has called SlideShare the YouTube equivalent for PowerPoint presentations.
Slideshare has released a new plugin for Microsoft Office 2007 that allows users to edit and publish presentations directly to their SlideShare accounts.

Beyond publishing new PowerPoint documents to the web, the SlideShare plugin can import SlideShare files from the cloud (both your own and those that are shared by others), which can then be modified on your native PowerPoint client. The plugin features an integrated search so users can browse through files from SlideShare contacts and groups, as well as support for Twitter and FriendFeed so they can broadcast a new presentation without leaving Office.