When it comes to atheism, theists tend to lump atheists together. Worse still, Christians apologists incessantly refer to "the atheist" in a creepy, nazirefic way as if atheism was some sort of monolithic ideology that all self-described atheists adhere to.
But not all atheists are the same. Atheism, like theism, is a broad category that reveals little about the specifics of what people believe. Atheism, like theism, encompasses a variety of different beliefs. Here are a few of them that I have noticed: (this is meant as a quick overview and not a comprehensive list)
Communism. I have to talk about this one because McCarthyism casts a long shadow and there are still people in the States who don't understand that there are atheists who aren't communists. But there are indeed atheists who do subscribe to Marxism-Leninism, especially in the few remaining communist countries.
Objectivism. The polar opposite of communism. Extremely pro-capitalism, reason as the only way of perceiving reality, and emphasis on rational self-interest as opposed to altruism.
Religious atheists. Believe it or not, atheism in and of itself does not preclude belief in the supernatural. It's quite possible to disbelieve in a creator god and yet subscribe to a religion.
Buddhism, especially Zen Buddhism, is often described as an atheist religion because god in the sense of an eternal creator god doesn't exist. Although devas are believed in, they differ from gods in the western sense of the word in very important ways - they aren't immortal or perfect and didn't create the universe. Buddhism differs from most western religions in that supernatural devotion is secondary compared to enlightening oneself and attaining nirvana.
LaVeyan Satanism can also be said to be atheistic. Adherents do not believe in any deities or any supernatural beings at all. Rather, the Satan of the Bible is a seen as a positive symbol of mankind's inherently individualistic, prideful, and carnal nature.
Many UFO religions can be correctly described as atheistic, albeit with powerful alien civilizations taking on roles traditionally assigned to Gods. Raelism is one such religion, with sufficiently-advanced aliens, the Elohim, who seeded the Earth with life (they're big Intelligent Design advocates), sent many prophets to spread their message (including Buddha, Jesus, and Muhammad), and will one day return to share their advanced technology with us. So where's the atheism? Raelians don't believe in souls or any gods. Rael himself took the Blasphemy Challenge and praised the Out Campaign, which gives me mixed feelings to say the least.
Secular Atheists. This is the big enchilada - atheists who not only object to notions of gods, but also to anything supernatural at all. Arguably, most self-described atheists fall in this category. These atheists tend to be nonreligious, highly value science as a way of knowing that is much more reliable than faith, apply skepticism to truth claims, and support secularism. And unlike communists or objectivists, many secular atheists tend to hold more moderate political views ranging from liberal to centrist to libertarian. Secular atheism has its roots in western religious skepticism, from Greek to Enlightenment to modern arguments against belief in a god. Secular atheists may subscribe, in whole or in part, to a wide variety of philosophies and ethics. I myself am quite fond of secular humanism, existentialism, and epicureanism.