Adobe Labs - Acrobat.com Presentations
http://labs.adobe.com/technologies/presentations/

* Easily create your own professional presentations online, using built-in tools and layouts. * Simplify working with others on presentations. Create, revise, and collaborate on the same presentation at the same time -- all online. No need to e-mail attachments back and forth or track down who has the latest version. * Meet your deadline with ease. With simultaneous editing, no one is locked out of the presentation while others make changes. * Access your presentations from anywhere. Your presentation is always available online so you can do last minute tweaks, present it from wherever you are, or deliver it offline by exporting to Adobe PDF.

Here are some light alternatives to the hefty Adobe Reader