I am not a big fan of writing for free. You certainly see many opportunities out there for you to do so. These nice folks are going to "help" you build up writing credits by writing for free. Usually, this is not a great idea. Beter to spend your time writing things that will sell.
That being said, I just wrote an article for free. Why did I do it? Simple. The website asked me to write an article and so I did. However, first I checked to see the site's Alexa and Google ranks. This is a big-time site.
You can see my article, How to Choose an Online High School, by going over to the About.com site. About.com is owned by the New York Times. As part of the deal, I get a bio page where I can set up inbound links to my online high schools website.
Those inbound links are anchor text (like you see above in those hyperlinks). That is exactly what you want if you are hoping to increase your page rank. Page rank means more money.
I guess what I am saying is that, if opportunity comes knocking, you may want to consider the ramifications.