The religion of the Sumerians. It is polytheistic, with a god or goddess representing forces or presences in the world much like Greek religion. Humans were originally created as servants for the gods, but were freed when they became too much for the gods to handle.
Many stories in Sumerian religion have analogues in other middle-eastern religions. For example, the biblical account of the creation of man as well as the flood narrative are very similar to Sumerian tales. Gods and goddesses from Sumer have distinctly similar representations in the religions of the Akkadians, Caananites, and others. There are a number of related stories and deities shared with the Greeks as well; for example Innana's descent into the underworld is strikingly similar to the story of Persephone.
Cosmology: The universe was created when Nammu, a presumably formless abyss, curled in upon herself, and in an act of self-procreation gave birth to An, god of heaven and Ki, goddess of Earth (commonly referred to as Ninhursag).
The union of An and Ki produced Enlil, lord of wind, who eventually became leader of the pantheon. After being kicked out of Dilmun (the home of the gods) for raping Ninlil, Ninlil has a child, Sin (god of the moon), also known as Nanna. During Enlil's banishment, he also fathers three underworld deities with Ninlil, the most famous being Nergal.
Nammu also gave birth to Enki, god of the watery abyss, or the Abzu. He also controlled the Me, holy decrees that governed such basic things as physics and complex things such as social order and law.
This accounts for the origin of most of the world as we know it.