A free product key (free CD key) for a Microsoft Office version could be really useful for several reasons.
For one, a free Microsoft Office product key would be great if you've lost your product key but you're wanting to reinstall Microsoft Office. A free Office CD key would really come in handy if you've borrowed a friend's copy of Microsoft Office 2007 or Microsoft Office 2003 and you'd like to install it on your own computer.
Many websites list product keys for Office software including Microsoft Office 2007, Microsoft Office 2003, and more. The CD keys these websites offer are likely created with a product key generator program or are duplicate product keys from legitimate copies of Microsoft Office.
It doesn't matter where the product key came from - using a product key other than the unique one that comes with an individual copy of Microsoft Office is illegal. Using a product key more than once is usually a violation of the software agreement you make with Microsoft when you use their software.
Requiring a unique product key is one way that Microsoft can be sure that each copy of their Office software is used only once and that you are paying for the copy you are using.
If you have a legitimate copy of Microsoft Office but you've lost your Office CD key, there are legal ways of locating your Office product key.
Sorry to break it to you, but installing a someone else's legal copy of Microsoft Office on your computer is not legal. A friend's CD key is only valid for his or her copy of Office. If you need Microsoft Office installed on your computer, you'll need to use a unique product key. The only legal way to obtain a unique product key is to purchase a copy of Microsoft Office.