Just to back up Fo3nix's post:
Open Office is generally regarded as being as good, if not better, than Microsoft Office. Many major companies and local authorities are switching to it, rather than continuing to pay Microsoft's licensing fees.
It's (almost) fully compatible with Microsoft documents so, for example, if somebody mails you a Word document, you should be able to open and edit it. (I had to include the word 'almost' in that sentence because Open Office might not be able to handle very complex Microsoft files which, for example, use complicated macros. Otherwise, there's no problem) and, of course, it's free
http://www.openoffice.org/
Chris