Considering the importance of agriculture in ancient society, it is not at all surprising that ancient pantheons pretty much always had at least one fertility deity. In fact, a lot of the earliest known gods fit into this category, and a few of them managed to retain their significance when religion began to incorporate other aspects of civilization. In Greek mythology, the most important fertility goddess was Demeter, daughter of Kronos and Rhea, and hence sister of Zeus, Poseidon, and Hades. She does not feature all that much in the heroic myths, but she had her own place in Greek religion. In addition to agriculture, she was also a patroness of marriage (which I suppose would have overlapped Hera’s position somewhat) and death and rebirth. She was also central to the Eleusinian Mysteries, which predate the establishment of the Olympian pantheon.
The most famous myth involving Demeter involves her daughter Persephone. Persephone was the child of Demeter with Zeus, and Hades abducted her to make her his bride. When Demeter finally found her, Hades allowed her to return to her mother. Due to an incident involving a pomegranate, however, she had to divide her time between the upper world and the Underworld. When Persephone is with her husband, Demeter goes into mourning, and nothing grows. That, according to the myth, is why the seasons change.
Other myths featuring this goddess include her attempt to burn away the mortal nature of Demophon of Eleusis, her liaison with Iasion, and her rape by Poseidon when they were both in the form of horses. Demeter’s Latin name is Ceres, from which we get the word “cereal,” as well as the name of a dwarf planet in the asteroid belt and the space station in Super Metroid.