Open Office is a free, open productivity suite. The download includes a word processor as well as software to create spreadsheets, presentations, and databases. The real boon for math-science sutdents and professionals is the software that can be used to write papers that involve lots of formulas.
Unlike Microsoft Word, Open Office is supremely compatible across different operating systems. It is also very easy to collaborate and share documents on Open Office. According to my father, Xingping Sun, the entire math department at Texas A&M is now using Open Office.