When I discuss atheism with other people, which does not happen often, they sometimes bring up ethics. Some people even question how it is possible for an atheist to behave ethically. Of course, such a question implies that the questioner only acts ethically because they must, perhaps through fear of hell or a desire to look good for god. I doubt this is actually the case for most people.
It's true, of course, that atheism by itself does not address ethical questions. Nevertheless, I think I'm a basically ethical person. I try to follow the Golden Rule—do unto others as you would have them do unto you—and, more generally, Kant's categorical imperative—roughly, act in such a way that anybody in the same situation could reasonably choose to act the same way. These are simple guidelines which are not sufficient for a fast moving world with imperfect information, but a full discussion of ethics probably requires a different essay.
I follow these guidelines primarily for two reasons. The first reason is a logical one: they tend to lead to a world in which I am more comfortable. If I choose to be mean to other people, then other people will be mean to me, and sometimes unpleasant things may happen to me. The second reason is an emotional one: it makes me happier to be around people who are happy to be around me, and the easiest way to achieve that is to be a nice guy.
I think that ethical behaviour in ordinary situations is no more complicated than that, and there is no need to import ethical rules from beyond. There are certainly morally ambiguous situations, where it is not clear what to do, but those situations arise for atheist and non-atheist alike (I don't know of a word which means the opposite of atheist; logically it should be theist, but that means something slightly different).
The only case I see where one must seriously consider ethical differences between atheism and non-atheism is temptation, in which you are strongly tempted to do something which you believe to be wrong. Temptation occurs to everyone, and most of us give in to it once in a while. If there were any evidence that non-atheists resisted temptation better than atheists, that might deserve examination—not that all atheists have the same ethical beliefs. However, I know of no such evidence.
Finally, I note that there is no contradiction between my desire to be free of god's rules and my choice to constrain my actions by following rules of ethics. If I were following god's rules, they would be imposed on me from without. Choosing my own rules is a free choice from within.