Windows: PowerPoint Viewer, saviour of those who need to open, but not create, Microsoft PowerPoint files, has been updated to open the PowerPoint 2010-formatted presentations you just know people are going to be sending soon.
If you don't plan to have Microsoft Office 2010 installed by mid-year (when it's slated for release), you'll want to hold onto PowerPoint Viewer. This version of the program opens up the new PowerPoint 2010 file format and includes all the bells and whistles, such as the new transitions and video effects of the next edition of the suite. There's a caveat: it won't support embedded online video.
Unfortunately, it's not a portable utility so you can't carry it on-the-go, but it's useful if you don't want to install all of Microsoft 2010. PowerPoint Viewer is a free download for Windows systems only. Are the new abilities in PowerPoint 2010 worth the upgrade, from what you've seen (or tried)?