Agamemnon's son; he kills Klytaimestra and her lover Aigisthos as vengeance for his father's death. When the gods meet at the beginning of the Odyssey, first on the agenda is the murder and whether it was warranted. This is no coincidence. See "Symbols, Imagery, Allegory" for more.
For more on this guy, check out Shmoop's guide to Orestes.