LibreOffice
LibreOffice from the Document Foundation is a free office suite that is a suitable replacement for Microsoft Office. The software is free, and works on all platforms.
LibreOffice is able to read and write documents in Microsoft Office formats; enabling you to share your work with people using Microsoft Office. There isn't 100% interoperability between the two, because Microsoft isn't interested in that. So, in some settings you may be forced to purchase Microsoft Office. LibreOffice also exports PDF documents with a single click; so if you simply want to publish a finished product, you can easily share a pdf file with others.
It is also based on a completely open file format so that your documents will never become hostage to the vendor. LibreOffice is a next generation version of OpenOffice which was taken over by Oracle Corporation when they bought out Sun Microsystems (the primary developer of OpenOffice at the time).
