This Web app provides a familiar set of tools for designing presentations. You can choose backgrounds, create text, and add images, audio, video and even transitions to your slides. mPower Play also lets you import existing PowerPoint presentations, though your export options are limited to HTML. Need help crafting the perfect presentation? You can invite unlimited collaborators or viewers.
Although mPower Play's interface seems a bit crude (Windows 3.0 comes to mind), I was able to whip together a presentation pretty easily. The finished product can be shared via e-mail as a standalone site, or hosted by mPower for a real-time "Webinar" with unlimited participants, attendance tracking, etc.
The free version of mPower Play limits you to 50MB of online storage and no Webinars; for $9.95/month you get 2GB of storage and unlimited webinars. If the idea of online presentation-building sounds appealing, make sure to check out Zoho Show as well.