LibreOffice is the most comprehensive and complete free software office suite available. It includes a word processor called Writer, a spreadsheet program called Calc, a presentation program named Impress and a vector graphics editor called Draw. There is also an application for creating and using mathematical formulas called Math and a database program called Base.
LibreOffice is available for GNU/Linux and all the distributions have it available in the default repositories. It is also available for FreeBSD, NetBSD, OpenBSD, Mac OS and Windows.
GNU/Linux users should install it using the regular repositories. Users of other operating systems can get it from it's homepage libreoffice.org.