There are very few examples, one of which is Aeneas:
a Trojan hero, the son of the prince Anchises and the goddess Aphrodite.
The Wikipedia article on demigods gives a good cultural explanation as to why there were not many:
The fact that male deities of Greek myth had far more notable children with mortals than the female goddesses can be attributable to the Greek male-dominated society being reflected in their religion. Zeus, primarily, and also Poseidon, both had a multitude of affairs with mortal women, with Zeus having to shield them from his wife Hera after she was alerted to the infidelity. The females were expected to remain loyal to their husbands, while the males were almost expected to take multiple lovers, meaning that far more of the demigods in Greek myths were born on earth to human mothers than on Olympus to divine mothers.