Sun Microsystems have now released v3.0 of their popular 'OpenOffice.org' productivity suite. It's a fantastic alternative for current users of Microsoft Office, or those who wish to have the functionality of the Microsoft Office suite, but do not like the high pricetag.
It has compatibility with all the Microsoft Office file types (including new .docx for Microsoft Word 2007)
Installation of OpenOffice.org is easy - follow the links below for walkthroughs:
For online only use, perhaps consider Google Docs - you can download and upload files from here also, meaning that at a push, you could use it to edit a file where you do not have your normal office suite installed.
There is a huge list of other alternatives at wikipedia - our preference definitely lies with OpenOffice.org however.