It can be a wrench to move away from Microsoft Office and onto free alternatives, but these days there are high quality, entirely free alternatives that work on a variety of platforms and operating systems.
Unless otherwise stated, each of the alternatives below offer a word processing, spreadsheet, presentation applications (and usually one or two others such as a drawing package or form-creator).
LibreOffice has been around for many years and spawned from the OpenOffice project so is based on firm foundations. It offers a free suite of the usual three applications plus a database manager, drawing package and charting/graphing tool, and works on Windows, Mac and Linux operating systems.
Google Docs is probably the best known of the freebie office suites, offering the usual applications plus a tool to create online forms and a drawing application. It also seamlessly integrates with other Google tools, meaning it gives you access to things like free online storage through Google Drive and mobile apps for use on other devices.
Similar to LibreOffice, Apache Open Office was borne from the original OpenOffice project. In addition to the usual tools, it includes a drawing and mathematical equations application. It also works on Macintosh, Windows and Linux. It too uses the Open Office document format but will also read many Microsoft Office document formats. At the time of writing, Apache Open Office claims over 100 million dowloads.