LibreOffice

Free office suites aren't many, and only one is a download for the major desktop operating systems. The open-source LibreOffice, now in version 7, suffers a bit in the polish department, lacks collaboration features, and sports an overstuffed toolbar interface that might remind you of Microsoft Office a decade ago. But it's powerful nevertheless, and it easily converts/imports files from other systems. It comes with a word processor (Writer), a spreadsheet (Calc), a presentation program (Impress), a vector drawing program (Draw), and even a full database (Base) and math-formula editor (Math). And it's all utterly and completely free.