One of the common accusations I hear from Christian apologists is that atheists have no basis for their morality. Admittedly, it's important to consider the source in situations like this. The most recent case that I'm aware of is Ken Ham, a man whose goal in life seems to be to condense the depth, richness, and variety of Biblical teachings into a handful of bumper-sticker slogans as a service to future generations. Since this Quixotic quest forces him to discard a significant portion of scientific knowledge (and, arguably, the entire scientific method) as incompatible with his grossly oversimplified approach to Christianity, he may conceivably not be the best man to consult on, well, much of anything. Nevertheless, the view that he presents - this idea that atheists have no firm basis for their ethics, and perhaps no ethics at all - is not uncommon among the more insular sorts of Christians.
So, strictly as an intellectual exercise, I thought I'd demonstrate how it's possible to get to a consistent basis for morality without any appeal to (or belief in) the supernatural. Here's how it works:
It's a fairly simple observation - I'd go so far as to call it obvious - that people get more done when they work together. We can set up systems: share resources, divide labor, concentrate on one task or one type of task instead of having to do everything for ourselves. We may not always enjoy it - there's friction, and disagreements, and power struggles - but it does get more done. So, in terms of making sure that people have both basic necessities and little luxuries, cooperation is highly beneficial. Cooperation, then, is why people tend to live in groups: tribes, towns, nations, and other social systems.
Those sorts of social systems work best, however, when everybody involved is playing by the same rules - and, I think, when those rules have provisions that allow them to adapt to new or changing information. That means, basically, that everybody shares the same set of morals. That never actually happens, of course, but any functioning social group will have a workable approximation: a social contract.
Part of the difficulty in establishing such a contract, however, is what project managers like to call "buy in". Basically, if you want to implement a new system, or change an existing system, you have to get enough people to agree in order to make the change work. And it's easiest to get people to agree voluntarily is if the system (or the change) treats everyone fairly (or more fairly). In other words, whatever the specifics of your social contract, it will work best if you treat other people the way you would want to be treated if you were in their place.
And there you have a working atheistic basis for morality: "Do unto others as you would have them do unto you."
This is enormously simplified from the details of actual situations in the real world, but it does (I hope) show how you can easily derive the Golden Rule from simple observation of human nature and the natural world.
 "Apologist" in this context usually means "evangelist" ... and not infrequently "asshole" as well. Note that this does not in any way typify the vast majority of Christians, most of whom - in my experience - have a great many more important things to occupy their attention.
 There are other ways to get people to agree, of course: fraud, coercion, threats... I'd argue, however, that over the long haul those are quite a bit less efficient than voluntary cooperation, and that they're easier to enforce when resources are scarce; where resources are more freely available, it's a lot harder to force people to do things that they don't want to do.
 The process of simplifying complex real-world difficulties into simple abstractions is, I believe, usually referred to as "Philosophy".