Freedom to Marry is "the leading campaign working to win marriage nationwide." They "partner with a diverse range of organizations and supporters across the country to end the exclusion of same-sex couples from the responsibilities, protections, and commitment of marriage." The founder, Evan Wolfson, was co-counsel in the historic Hawaii marriage case, argued before the U.S. Supreme Court in Boy Scouts of America v. Dale, served in the Peace Corps in West Africa, and is the author of Why Marriage Matters: America, Equality and Gay People's Right to Marry.
http://www.freedomtomarry.org/ has articles from all over the world and an interactive US map, where you can click on a state and be shown the status of equal marriage, what organizations are actively working on equal marriage, and related blogs and resources.