1.  OpenOffice
     OpenOffice.org is both a product and an open-source project. Both have been in existence since October 13, 2000. OpenOffice.org 1.0, the product, was released on April 30, 2002. OpenOffice.org's Mission Statement is...
To create, as a community, the leading international office suite that  will run on all major platforms and provide access to all functionality  and data through open-component based APIs and an XML-based file format. 
2.  KOffice 
     The KOffice project, being part of the KDE community, is dedicated to  producing Free Software, therefor you can download and install KOffice  in most cases free of charge. While the KOffice project only provides  source code, it is easily installable through the package manager of many linux distributions, FreeBSD.
3.  GnomeOffice 
     GnomeOffice. There are a bunch of GNOME/Gtk applications that are useful in an office enviroment. 
4.  Siag Office 
     Siag Office is a tightly integrated, free office package. It consists of  the spreadsheet Siag,  the word processor PW, the animation program Egon,  the text editor XedPlus,  the file manager Xfiler and  the previewer Gvu. 
5.  LaTeX 
     LaTeX is a high-quality typesetting system; it includes features designed for the production of technical and scientific documentation. LaTeX is the de facto standard for the communication and publication of scientific documents. LaTeX is available as free software. 
6.  TeX 
     The TeX Users Group (TUG) was founded in 1980 to provide an organization for people who are interested in typography and font design, and/or are users of the TeX typesetting system invented by Donald Knuth. 
7.  AbiWord 
     AbiWord is a free word processing program similar to  Microsoft® Word. It is suitable for a wide variety of word processing tasks.
8.  StarOffice 
     StarOffice is the direct competitor of Microsoft Office as an office suite. The program has been developed by Sun Microsystems for many years, being the pay version of the open source project OpenOffice.