There is also a open source version of PowerPoint., Open Office.
OpenOffice.org 3 is the leading open-source office software suite for word processing, spreadsheets, presentations, graphics, databases and more. It is available in many languages and works on all common computers. It stores all your data in an international open standard format and can also read and write files from other common office software packages. It can be downloaded and used completely free of charge for any purpose.
The Open Office versoin of PowerPoint can do everything that the Microsoft PowerPoint can do. You can export the file to a .pps or .ppt format so someone with Microsoft's Powerpoint can open it.
Get Open Office here.