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.
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.
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.
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.
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.
AbiWord is a free word processing program similar to Microsoft® Word. It is suitable for a wide variety of word processing tasks.
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.