The Freedom From Religion Foundation, as quoted on their website, http://www.ffrf.org/ , “… with more than 13,000 members, is the largest association of freethinkers (atheists and agnostics) in the United States [of America].”
I really like the newspaper they publish, Freethought Today, which I’ve been an on & off subscriber to, when I can afford it. The website offers “Freethought Of The Day” which usually notes the birthdays of wellknown atheists, agnostics, or freethinkers with short bios and quotes.
They also have a radio talk show, conventions, student essay competitions, etc. & help fund legal battles to uphold the separation of church & state, as well as put up billboards, & go on other advertising campaigns.
Freethought Hall in Madison, Wisconsin, U.S.A. holds the national offices for the organization, FFRF is a non-profit tax-exempt organization.