OpenOffice.org (also known as OOo or OpenOffice) is an open-source office software suite for word processing, spreadsheets, presentations, graphics, databases and more, and runs on multiple platforms. OpenOffice.org originated as StarOffice, an office suite developed by StarDivision and acquired by Sun Microsystems in August 1999. The source code of the suite was released in July 2000 with the aim of reducing the dominant market share of Microsoft Office by providing a free and open alternative; later versions of StarOffice are based upon OpenOffice.org with additional proprietary components. The OpenOffice.org project is primarily sponsored by Oracle Corporation (initially by Sun Microsystems).
OpenOffice 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.
OpenOffice is also easy to use with similar keyboard shortcuts to Microsoft Office.
Update: OpenOffice has now moved to LibreOffice. So this post is deprecated.