There are a great many minor (and major) gods and goddesses in Greek mythology that were not part of the main group of twelve. Here are a few examples, but see the 'related links' for more.
Hestia (she was originally part of the twelve Olympians, until she gave up her spot for Dionysus)
Eros (Aphrodite's son)
Iris (goddess of the rainbow)
Persephone (the wife of Hades and the daughter of Demeter; goddess of springtime/plants)
