Catholic Answers Forum always has a handful of atheist posters from what I remember. You can have discussions and debates there so long as you are respectful. If they get the sense that you're just on there to troll at/bad mouth Christianity, they will ban you.
"Of course, everyone will claim they respect someone who tries to speak the truth, but in reality, this is a rare quality. Most respect those who speak truths they agree with, and their respect for the speaking only extends as far as their realm of personal agreement. It is less common, almost to the point of becoming a saintly virtue, that someone truly respects and loves the truth seeker, even when their conclusions differ wildly."
-walsh
-walsh