The three main freebies are OpenOffice
https://www.openoffice.org/
LibreOffice
https://www.libreoffice.org/
and WPS Office (formerly Kingsoft Office)
http://www.kingsoftstore.com/kingsoft-office-freeware.html
They're all excellent but OpenOffice is by far the most well-known (which means that, should you need it, there are loads of people here who can help you with it).
By default, OpenOffice saves files into its own formats, which aren't compatible with Microsoft Office. (So, if you send such a file to someone with Microsoft Office, he won't be able to open it). You can get around that problem easily enough by remembering to use 'Save As', instead of 'Save', and then selecting a Microsoft format. However it's probably better to change the default setting so that 'Save' automatically uses Microsoft formats anyway. There are easy instructions for doing that here:
http://helpdeskgeek.com/office-tips/set-openof%EF%AC%81ce-org-to-save-in-microsoft-of%EF%AC%81ce-format-by-default/