Metaethical Theories

Theists occasionally claim that atheists pragmatically 'believe' in God, because they act morally, or attribute praise and blame, which acts implicitly accept the existence of God. Behind this is the idea that an objective morality can only be grounded in God. Here's William Lane Craig:

...if God exists, then the objectivity of moral values, moral duties, and moral accountability is secured, but that in the absence of God, that is, if God does not exist, then morality is just a human convention, that is to say, morality is wholly subjective and non-binding. We might act in precisely the same ways that we do in fact act, but in the absence of God, such actions would no longer count as good (or evil), since if God does not exist, objective moral values do not exist. Thus, we cannot truly be good without God. On the other hand, if we do believe that moral values and duties are objective, that provides moral grounds for believing in God.

..and here's an article on Catholic website Strange Notions:

Objective morality is observable apart from knowledge of God, which is why atheists and agnostics can know right from wrong, and why philosophers can talk about self-evident moral propositions, and why everyone reading this knows what we mean by “moral” and “immoral.” Some things are just wrong, regardless of our philosophies, and even if we desperately want them to be right.

But objective morality isn’t explicable apart from knowledge of God: every attempt... fails to explain why objective morality exists.

But there are a number of theories that attempt to explain the grounding of ethics that have nothing to do with God; the Metaethics article at the SEP has an extensive bibliography that is mostly concerned with secular explanations.

Furthermore, God grounding is not a fruitful avenue for analysis, because that is where the buck stops, and that is that. (As such, it may be subject to Moore's Open Question argument.) But to get an idea of what is happening in contemporary metaethics, take a look at this diagram from Alexander Miller's An Introduction to Contemporary Metaethics (2003 edition):

Here we have a handy guide to the current debate in metaethics, little of it concerning God. God doesn't even warrant an entry in the index, while Peter Railton has 20 references! Here are links to some of those theories:

Ayer's emotivism
Blackburn's quasi-realism
Gibbard's norm-expressivism
Mackie's error-theory
Moore's non-Naturalism
McDowell's non-Naturalism
Railton's reductionism
Cornell realism

This does not show that morality is not grounded in God, of course, but it does show it would be wrong to suggest that that there are simply no alternatives for the atheist seeking a metaethics for her moral behaviour.