Apache OpenOffice Impress

Impress, part of the Apache OpenOffice suite, is a presentation program similar to Microsoft Powerpoint. It does dual screen and linear presentations out of the box, but the free "Presenter Console" extension adds much better dual screen and non-linear presentation capabilities. Impress works very well with existing Powerpoint files. It can save to various formats including Powerpoint, Open Document Presentation format, Flash (SWF) and PDF.