The Non-Olympian Gods
It should be noted that not being an Olympian God does not, in reality, lessen their status as Gods. Indeed, the Greeks worshipped a multitude of aspected deities which were different from region to region. In most cases, we are talking about different aspects of the Great Gods, but in others we are talking about powerful divine forces in their own right.
Perhaps highest among these are the mighty King and Queen of the Underworld, Brother and Daughter of Zeus, respectively, as well as powerful beings such as the Carian Goddess Hekate and the mighty all seeing Helios, god of the sun. These Gods may have been called different things in different parts of Greece, but those names that became known throughout all of Greece are the ones that became the standards. And in different parts of Greece, these Gods were seen with a variety of aspects and bearing different levels of importance. Where Hekate was a "Great Goddess" in Caria, she was a protector against witchcraft in Athens. Where the great Helios once inspire a colossal statue at Rhodes, he may not have been very important at all in Argos or Sparta.
Yet these gods were worshipped throughout Greece, whether by large cults or small makes no difference.