Some [UU churches] actively welcome atheist members.

I visited the uucpa once. Fifty people or so gathered in a room to sing songs for an hour; someone lit a flame, and rang a bell; and a congressperson gave a sermon on the health care reform bill. Afterwards, we went out for brunch. There is a humanist reading group as well.

I can see how an atheist would be comfortable there. It didn't personally excite me, though.