OpenOffice is a free office suite. It is actually called OpenOffice.org for some reason. It is based on Sun's StarOffice but it is released under the LGPL.
More info here
Aparently its called OpenOffice.org because someone else holds the copyright on the term 'OpenOffice' (although openoffice.com doesn't go anywhere) -- AlastairPorter
33 pages link to OpenOffice: