LibreOffice is a free and open source office suite developed by The Document Foundation as a fork of OpenOffice.org. It is largely compatible with other major office suites, including Microsoft Office, and is available for a variety of platforms.
LibreOffice can import and export documents in several file formats; its native format is the OpenDocument Format (ODF). Formats supported (for reading and writing) include the Office Open XML specification used in Microsoft Office 2007 and 2010 (.docx, .pptx and .xlsx), as well as to the older file formats used in Microsoft Office 95 and 97-2003 (.doc, .ppt and .xls). It also supports Rich Text File format (.rtf) and OpenOffice.org XML format. All documents can be exported to the PDF format, as well as presentations to Adobe Flash (SWF). LibreOffice also has the ability to import documents in read-only mode in the Uniform Office Format, Data Interchange Format and the formats of Microsoft Works, WordPerfect and Lotus 1-2-3.
(Excerpted from Wikipedia.)