LibreOffice. An open source alternative to Microsoft Office
Even though this is a fork of OpenOffice (a suite of applications developed mainly by Sun Microsystems and later abandoned when Oracle bought the company, now again in active development by the Apache Foundation), LibreOffice has superseded its ancestor offering better usability, ease of use and stability. It includes Write (a word processor), Impress (a presentation creator), Calc (a spreadsheet editor) and Draw (a graphic editor), and can take the place of Microsoft Office for most tasks. It’s available for Windows, Mac and Linux.
Here’s a review of the last major revision, version 5: