I find something like Ken Macleod's so-called True Knowledge tempting, at least as a fall back position (as the author of that page says). In the absence of a God, or at least, of one who cares enough to show herself plainly, what matters to you is what matters. If love, charity and loyalty are important to you, you should act to advance them as far as you can. Morality is whatever you can get away with, where "getting away" doesn't necessarily mean swindling people (although it might, there being no absolutes here), but merely advancing things which you consider good. If enough people agree with you, the power of your argument grows.
It's not clear the MacLeod himself thinks the True Knowledge is a good thing (believers in it commit what I'd regard as genocide in his book The Cassini Division). I found some interesting discussion of quite what it is he does think of morality, which makes reference to the True Knowledge idea. Graydon's views seem particularlty apposite.
Isn't this the bad old "might makes right" philosophy, which, taken to its logical extension, will lead to us driving around on smoke-belching killing machines while wearing leather and listening to Tina Turner? I'd like to hope not.
My own morality is based on my long term self interest. Being content, finding things out, having friends and loves are all things I enjoy, so I act to maximise my chances of such things persisting. That includes being part of a group and of a society which will allow such things to continue. I'm surely not unique in thinking that the Mad Max war of all against all isn't going to help further my aims. As Graydon says, power rests of peace.
In Greg Egan's rather good Distress, there's an artificial island which floats unsupported in the middle of the ocean (that's like, a bleedin' metaphor for existence, you see: keep up at the back). The society on it is one of your standard SF capitalist anarchies, although a little less hard nosed than the sort of thing you find in other authors' works. The response to the narrator's question "Why doesn't someone try to exploit the system and take over?" is amused condescension from the islanders, who point out that that the question amounts to "Why don't you all try to make your lives as miserable as possible?"
That's not quite the full story, of course: there are bad people out there, so whatchya gonna do when they come for you? The book's islanders have their own answer, which I won't spoil. The sort of biology based ethics which MacLeod seems to have in mind advises being nice to everyone you meet and walloping them if they wallop you. There are people who've not worked out that it's in their own best interests not to be an asshole, but eventually everyone else will move out of their way and defend their walls. I've made comments about burbclaves and things of that nature before. As I've been talking about Neal Stephenson, what I'm thinking of is more of a phyle (from his book The Diamond Age) than a burbclave, really. It's not about living with people who all like ballroom dancing, but rather about living with people who share your outlook.
Surfing around the other day, I found a Christian response to Bertrand Russell's Why I am not a Christian, in which the author points out that saying Jesus' belief in hell was immoral presupposes some sort of morality. The same goes for the Problem of Evil: where does your definition of evil come from? In both cases, my answer is that morality comes from within me by the process of considering what I want my life to be about. Ignoring Calvinism (which has huge moral problems of its own), it is my choice whether or not to accept Christianity's morality. I find the infinite torture of relative innocents to be unjust. There are other people who can accept this idea, and so be a part of the Christian phyle, but I'm not one of them.