LibreOffice
Despite the fact that Microsoft Office is among the most common commercial software products in most offices around the world, there's a wide variety of governments, organizations, and offices that have begun standardizing on the free and open-source LibreOffice. It provides many of Office's features, in software you can install on your computer (unlike Google Docs and other online alternatives), and it runs on Windows, OS X, and Linux. Plus, it supports the same file formats as Office.
LibreOffice gives you the full package including Writer - the word processor, Calc - the spreadsheet application, Impress - the presentation engine, Draw - drawing and flow-charting application, Base - database and database front-end, and Math for editing mathematics. That covers almost everything Microsoft Office includes. The only problem left is that its user interface can be somewhat confusing to start using.