Impress, a part of the OpenOffice suite of tools, is a great open source alternative for creating presentations. It is available for Windows, Mac and Linux operating systems and also has ports for other operating systems. Documents created using Impress can be saved in several formats, including the OpenDocument Format and as a PowerPoint file. Versions of Impress are available from both Apache OpenOffice and LibreOffice.
If you think Impress might meet your needs, watch this video from Lynda.com to see how it works. (Note: Because there are two different versions of Impress, one from OpenOffice and one from LibreOffice, some details may differ from the tutorial below.)