I'm interested in hearing what everyone's favorites are.
I'll start off the discussion by suggesting two of my favorite philosophers, both of whom were atheists and dealt heavily with how to live absent God: Albert Camus and Friedrich Nietzsche.
Camus is an existentialist who essentially asks us how we live in a world without objective meaning and objective truth. He deals with how we live and what we mean without a God to dictate our meaning.
A few favorite Camus quotes (yes, this is shameless promotion, but I think everyone should read him):
Albert Camus
I chose justice in order to remain faithful to the world. I continue to believe that this world has no ultimate meaning. But I know that something in it has meaning and that is man, because he is the only creature to insist on having one. This world has at least the truth of man, and our task is to provide justification against fate itself. And it has no justification but man; hence he must be saved if we want to save the idea we have of life. " (From Resistance, Rebellion, and Death, which is a collection of things written by Camus during the Nazi occupation of France [Camus is French])
Albert Camus
...Certainty is not gaiety of heart. We know what we lost on that long detour; we know that we are paying for the bitter joy of fighting in agreement with ourselves. And because we have a strong sense of the irreparable, there is as much bitterness as confindence in our struggle. The war didn't satisfy us. We had not yet assembled our reasons for fighting...This war is the one they chose for themselves, instead of accepting it from idiotic or cowardly governments, a war in which they recognize themselves and are fighting for a certain idea they have formed of themselves. But this luxury they permitted themselves costs a dreadful price." (Also from Resistance, Rebellion, and Death)
Nietzsche is an old favorite, too. He's the one famous for proclaiming that God is dead. Indeed, one of his characters goes on to say that not only is God dead, but we are the ones who have killed him... (Of course he means it metaphorically, to say that the old order of Christian morality is finished, not that God used to exist and then we killed him.)
Nietzsche primarily writes about the affirmation of suffering and the affirmation of life, but the passionate rejection of religion and religious moral systems are a huge part of his writings. Agree or disagree with him, he's an interesting read...
Thoughts?