I wrote about Google's PowerPoint viewer a while back and it looks like it's been released for everyone. I opened an email with PowerPoint attachments and found the below.
Clicking on "View as a slideshow" opens a slide viewer that's pretty slick (picture at right from Mashable).
You'll note from my example above that PowerPoint 2007 files (with the pptx extension) are not yet supported. Google was also unable to open one file that I had downconverted from the 2007 format to the 97-03 format. If you use PowerPoint 2007 keep these two things in mind.
As long as your browser supports Flash you can use a deck of slides anywhere.
This is really a nice function that will surely come in handy. In this incarnation it's not exactly a threat to PowerPoint's dominance in the presentation software space, but it is a sign of things to come.
Tip of the hat to Mashable