Barry Ritholtz asks "Who Are the Most Influential Liberal Thinkers?"
I would eagerly second many of the suggestions already made in the comments:
as well as contribute these:
update (1/4 @ 10:01pm):
Lest anyone accuse me of ignoring theists...I omitted a group of religious thinkers from this list, a problem which I will now rectify:
John Shelby Spong