I agree that OpenOffice.org is the best MS Office suite alternative out there. One thing to keep in mind is that it doesn't just replace Word, but also Excel and PowerPoint. Which is good if you want all of that, and bad if you only want to install a word processor.
There are a couple of areas that MS Office is better, though. OpenOffice takes a lot longer to start up. OpenOffice doesn't have a grammar check. The .doc files it creates are much larger than the same file from Word, and sometimes it doesn't appear exactly the same when you open it in Word. But aside from these minor things, it's pretty kick A.
And to go against the flow here, you also might want to check out AbiWord
. It's a pretty good word processor, too. It's a lot lighter on the resources, but also a little lighter on the features. And it's only the word processor without all of the other stuff. It also has export support for MS .doc format.