LibreOffice is a suite of programs for word processing, spreadsheets, presentations, databases, and vector drawings. It is free to download, use, and distribute. It is available in many languages and runs on Linux, Mac, and Windows.
LibreOffice’s native file formats are OpenDocument (*.ODT, *.ODS, *.ODP, etc.), but it can open and save to many other formats, including Microsoft Office formats (*.DOC, *.XLS, *.PPT, etc.).
To download LibreOffice, go to the LibreOffice website.
LibreOffice is a project of the not-for-profit organization, The Document Foundation.
A short introduction to the software is Introducing LibreOffice (PDF). For more detail, get the full book Getting Started with LibreOffice (see links in lefthand sidebar).