SlideShare

Think YouTube for for PowerPoint shows. SlideShare allows you to upload your PPT file (up to 20MB), tag, share, and play. There's no way to make slideshows private but the concept is interesting, especially for those who want to share their slides and don't have server space to post them on. The system is currently in private beta (which also limits viewing slides to those with accounts) but I've got invites. Drop me an e-mail if you want access.