Well…Kant seems to be the obvious one. I mean, he’s not actually an atheist but he did construct his system of morality as if he was one
Emmanuel Levinas, while not exactly a militant atheist, does a lot of work in locating morality in the human rather than the divine domain – as far as I understand his stuff (it’s tough going, to say the least), it’s the mere fact of ‘other being’ – our position in relation to that which of the same world as us, and yet not us, which generates a sort of ‘infinite obligation’. ‘Entre Nous’ is one of his classics, and has a translated Routledge edition.
A big fan of Levinas who is a militant atheist (he’s openly said that religion of any sort is utterly incompatible with philosophy) is Simon Critchley. I’ve not read his latest, ‘Infinitely Demanding’, as yet but it’s apparently a pretty challenging work on the obligation to political action
Hope that’s helpful…