Atheism is by definition the lack of a god or gods. This can be hard (I believe there are no gods) or soft (I do not believe there are gods). But that's what atheism is.
Beyond that, there are many atheistic philosophies, some of which have concrete moral philosophies.
Note, however, that Humanism is not necessarily atheistic. Many Humanists are also atheists, but there are Christian Humanists as well, and I'm sure every religious tradition contains its humanists. My best friends are Christian Humanists, though their Christianity is Jesus-centered rather than Pauline or Trinitarian or church-centered.