LibreOffice Writer and Productivity Suite
This is a fork of Oracle OpenOffice (where the project splits away with a competing product based on the same original code). It appears to be more innovative and progressive than OpenOffice and stores documents in native docx format. It will install on Linux, Windows and Mac OS X. It is free for use so no payment or licensing required. See more information at www.documentfoundation.org/download/. It is also considered to be a much easier progression from MS Office 2003 than to MS Office 2007.