libreoffice

According to Wikipedia:

LibreOffice is a free office suite compatible with Microsoft Windows, Mac and GNU/Linux. It has a text processor (Writer), a spreadsheet (Calc), a presentations creator (Impress), a databases manager (Base), a vector graphics editor (Draw) , and an editor of mathematical formulas (Math).

To the previous definition, I would add that it is also compatible with BSD OS (FreeBSD, OpenBSD, NetBSD).

LibreOffice covers a wide range of office needs and therefore can replace proprietary suites such as: Microsoft Office.