Obviously there's been a LOT of discussion here before my arrival to this thread, but just in case this hasn't already been mentioned...
Personally, I'm fairly convinced that Nietzsche was the only true atheist -- at least of those whose writings and thoughts we're aware of. From what I can tell, he's the only one to have genuinely come to terms with the fact that if God is dead, then there is no firm, objective basis on which to determine right from wrong; that, in fact, such concepts as 'right' and 'wrong' are in the end meaningless. Rather, man must go 'beyond good and evil', and decide that which is of most benefit to oneself. (Actually, not 'man' generically, but rather the ruler of men must go beyond good and evil.)
Of course all cultures and civilizations have formulated their own moral or ethical codes, but we don't have to scratch too far below the surface to find that, as a result, these rules made life quite ruthless, nasty, and mean. They all naturally benefited the powerful ruling class most of all, and those who claim that the disparity between the few 'haves' and the many 'have nots' is greater now than ever before, obviously don't know very much of history. (In point of fact, the gulf between the two is smaller now than ever before -- and that by a long shot.)
Truthfully, the world today is so thoroughly saturated with the influence of the Judeo-Christian ethic, that we cannot even begin to appreciate what life would be like without it. Atheists can claim all they like that, without God and the Bible, our ethical standards would be at least as high, if not even higher. In his debates with theists, Christopher Hitchens likes to challenge his opponents to come up with a moral precept that could not have been arrived at through reason alone. He then enjoys it even more to declare that no one has come up with one yet.
First of all, it's a nonsensical demand from the outset: the theist is being asked to prove a negative.
Second, naturally there are logical reasons behind all ethical precepts. But it remains that not all were implemented apart from in those areas influenced by divine revelation. E.g. there are certainly sound logical reasons behind the sabbath day of rest, but just as certainly this principle was not implemented anywhere else but within Judeo-Christendom.
There are logical reasons behind recognizing the social equality of both males and females, but it's not by accident that it was only in nations with a significant Christian heritage that such a value was ever enshrined. If there is anywhere else in the ancient world where can be found words to the effect that "God created humankind in his own image, in the image of God did he create it, male and female he created them" (Gen 1.27), I have yet to see them.
Another example would be, in every ancient law code that I've come across, there are demands that anyone who finds a fugitive slave is to capture them and return them to their owner. The punishment prescribed for failure to comply with this was no less than death (and a small reward was offered to those did so). This is in stark contrast to the teaching of Deuteronomy 23.16-17, which requires: "If a slave has escaped from his master and taken refuge with you, you are not to hand him back to his master." Not only that, but it further commands: "Allow him to stay with you, in whichever place suits him best among your settlements; do not mistreat him."
All things considered, the Bible's moral codes are positively enlightened when compared with all others.