About Free PowerPoint Viewers

A PowerPoint Viewer is a piece of software, that usually comes as an executable file, that you can download and install on your computer so that you can view PowerPoint files (usually with the extension .ppt)
A PowerPoint Viewer lets you view full-featured presentations even if you don't have the Microsoft Office or PowerPoint installed on your desktop pc, laptop or mac.
Most of the free PowerPoint Viewers also support opening password-protected MS PowerPoint presentations. You can view and print the presentations, but you usually cannot edit them in the PowerPoint Viewers.
Free PowerPoint Viewers enable you to share PowerPoint presentations with users who do not have PowerPoint installed on their systems. The PowerPoint Viewers allow you to view and print PowerPoint presentations, but they do not allow you to edit the presentations.
In most of the cases the PowerPoint Viewer must be installed separately from the other applications — it is not installed by Microsoft's Office Resource Kit Setup program or by OpenOffice Setup thingie.

PowerPoint 98 Viewer for Mac

With this (again) free viewer you can view .ppt or .pps files under Mac OS9.x or Classic under OSX.
Free download here.