God

A God is a supernatural, immortal being believed in or respected by believers. While the existence of one or more Gods remains subject of debate in the real world; Gods and God-like aliens have often being depicted as real entities in science fiction; whilst sometimes non-"supernatural" aliens will make use of superior technology to become worshiped as gods by more primitive cultures. It is also possible to equate the two situations using one of Arthur C. Clarke's laws that states that any sufficiently advanced technology is virtually indistinguishable from magic. The belief in pantheism consist of the idea that the Universe itself is God.

"Real" and Mythological Gods

 * Cthulhu (more correctly a demi-god, as a Great Old One), worshiped by the Spawn of Cthulhu (At the Mountains of Madness) and by Humans (The Call of Chthulhu)
 * Several other entities collectively known as the Great Old Ones (demi-gods) and the Outer Gods (such as Yog-Sothoth and Azathoth) in H. P. Lovecraft's fiction
 * The entity from Robert Sawyer's Calculating God
 * The Firstborn from Arthur C. Clarke's Space Odyssey series
 * The Hitchhiker's Guide to the Galaxy series by Douglas Adams mentions God as a real entity, though noticing that the existence of the Babel Fish provides pretty strong evidence against him, and that he may have already vanished in a puff of logic. Also mentioned in the series are: Almighty Bob (worshiped by the Lamuellans), the Great Green Arkleseizure (worshiped by Jatravartids), Rob McKenna (worshiped by the clouds), and Thor, God of Thunder (in addition to the other Asgardians and Olympian gods)
 * Barry is a worshiped by the Melmacians. Apparently, after their 233rd birthday all Melmacians must chose between becoming ministers for the cult of Barry or becoming outcasts

Fake Gods

 * An unnamed evil entity at the center of the Milky Way Galaxy claimed to be God in Star Trek V: The Final Frontier
 * Q also claimed to be God in the Star Trek: The Next Generation episode "Tapestry", where he guides Picard in his near-death experience
 * The Dalek Emperor was worshiped as a god by Daleks who survived the Last Great Time War in Doctor Who
 * Yautja were treated as gods by ancient South American cultures on Earth in Alien vs. Predator

See Wikipedia's article on God- God