This is one of the most frequently asked questions on AB!
The most popular alternative here is, without a doubt, OpenOffice:
https://www.openoffice.org/
It's what I use myself. If you decide to follow suit, I'll give you a useful hint:
By default, the 'Save' function in OpenOffice uses the programs own format, which isn't compatible with Microsoft Office programs. (So you can't then email your documents to people who're using Microsoft Office, as they won't be able to open them). It's easy to get around the problem by simply remembering to use 'Save As', instead of 'Save', as that allows you to choose a Microsoft format. However it's probably better to change the program's default operation, so that using 'Save' automatically uses a Microsoft format. Instructions here:
http://helpdeskgeek.com/office-tips/set-openof%EF%AC%81ce-org-to-save-in-microsoft-of%EF%AC%81ce-format-by-default/
The two main rivals to OpenOffice are LibreOffice
https://www.libreoffice.org/
and WPS Office (formerly called Kingsoft Office):
http://www.kingsoftstore.com/kingsoft-office-freeware.html
Both have fans among ABers.
All of the above programs are full office
suites, which include spreadsheet, database and presentation software. If you ONLY want a word processor, AbiWord is also popular with some people here:
http://www.abiword.org/