After being raised Methodist, I stopped attending church during college and have attended Quaker services for a month since then. Not what I need at this stage in my life, but I can see myself going back. UU is another good option.

It seems to me that the shared activity of "listen to a lecture and sing some songs" is one that doesn't have the same appeal when you strip it of its religious overtones. (Especially the Quaker version, which omits the lecture and the songs.) I suspect a philosophy meetup or book club would serve the same purpose for most atheists, and if the rationality dojos ever get off the ground they strike me as another strong substitute.