More than a mere rival to Microsoft Office, the open-source OpenOffice.org (for Windows and Mac) includes the powerful applications for creating text documents, spreadsheets, presentations, diagrams, and databases that make it a serious Office competitor. It lets you edit basic documents and save them in a variety of formats, it's robust enough to handle equations and complex multipart documents with bibliographies, reference tables, and indexes, and on top of all that, it's extensible and uses less RAM than MS Office. What's not to love?