PowerPoint Viewer v1.5 (ActiveX Control)

PowerPoint files (.ppt, .pps) viewer from Viscom Software that also lets you export ppt files to image files like JPG, BMP, TIF, PNG, GIF.
Version 1.5 has added GetSlideCount method and the Quit method.
You can download it from here.

Google PowerPoint Viewer

If you have a Gmail account and you get a mail with a Microsoft PowerPoint Presentation you can view it without having MS PowerPoint installed or any other free viewers. You simply have to click the "View as slideshow" option under the attachment.
Ta-daaam!
:P

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.

PowerPoint Viewer 2007

This free Power Point Viewer is an updated version of the PowerPoint Viewer 2003 built for Windows 2000 Service Pack 4; Windows Server 2003; Windows Vista or Windows XP.
Free download here.

Free ppt viewer from Microsoft: PowerPoint Viewer 2003

This PowerPoint Viewer fom Microsoft Office is totally free and lets you view full-featured presentations created either in PowerPoint 97 and later versions.
You can download for free this viewer from here.