How can I find someone's address for free?

I am trying to find someone's address and I have their social security number, as well as name and other relevant stuff, but they have moved and I don't think that their address in the white pages is up to date. I've looked at a few other websites but all of them require a fee of some sort, and I'm not ready for that :P How can I get their address for free and to where it is up to date?