The generation of a moral code without recourse to some omnipotent deity raises lots of very interesting questions, but I think Carl Sagan came closest to a solution in my opinion (not that I've read them all of course).

Sagan stated (and I am paraphrasing freely here) that human beings are the universe made conscious - we are the cosmos able to contemplate and analyse itself, to divine it's own true nature and we are driven to understand the motion and actuality of matter and energy, of time and space. Far from removing the sense of wonder from our perceptions of the world around us, science reveals the true

Here is the basis of a moral code: every being is precious, capable of incredible insight and creativity. Every being is born with an equal right to exist alongside all others - the very fact you have been born at all means you are a unique, special and deserving of you place in the scheme of things, to realise your own potential.

So by killing or inflicting suffering on another human being (leaving out everything else for the sake of simplicity), you are in effect committing a crime against the very universe itself. You take away the real potential of another human being and deprive the cosmos itself of another piece of it's own consciousness, of it's own ability to reason and discover and abstract.

If you agree with this concept then constructing a moral code becomes a dialectic matter and very possible (most of the work has been done after all) - and the moral vacuum all the religious types seems to think would engulf us itself becomes less of a threat.

See? Don't need a God after all. Plenty of space for wonder and spirituality, compassion and empathy - and you take the responsibility yourself.