Now don't get me wrong, I think that Microsoft Office has the greatest marketshare because it's the best office package available. it works, works well and for a lot of people it's well worth the price especially with Microsoft Office Home & Student available at historically low prices.
However, not everyone needs all of the features or the expense of Microsoft's offering and some people just like to see Bill Gates have some competition. So, here are two terrific alternatives and they are both FREE!!

Openoffice.org is one of the main competitors and for good reason. it's got a ton of features, is used by many businessses and some governments and has all of the components needed for a complete office suite; a word processor, a spreadhseet, a presentation program, a graphics program and a database, all opensource and free.
For more information go to:
http://why.openoffice.org/The next alternative is a good replacement word processor for those not needing a full suite:

AbiWord is a free word processing program that works much like Microsoft® Word and can open many Word documents without problems. You can also save documents you create in Abiword in Microsoft® Word format so that your friends who have Word won't have any problems opening your files.
It also has a variety of dictionaries available in many languages, including British and American English dictionaries, which are great for the Canadian linguitic purists (and I'm sometimes one of them) who hate seeing labour spelled without the "u" and councillor with one 'l."
Give it a try - it works well. More information at:
http://www.abisource.com/
Put your files into your Dropbox on one computer, and they'll be instantly available on any of your other computers.