Refinance mortgage offers, "no costs to you" are these legit? - best mortgage offers

Should I refinance my mortgage soon. For the last 6 months or so my mail has been offering a daily basis with offers from different mortgage brokers or lenders to me a contract for life under water. Many say they offer me the financing at no cost to me. This seems a bit risky, I do not think anyone is willing to help free of charge. Is there any truth to these offers? I do not want to waste time in contact with these companies to take responsibility, if not satisfied. Ideas? Thank you!