Dead parents, like battle mechs and nosebleeds, remain a common plot device in many anime series. Some parents die in spectacularly dramatic ways, like dying in explosions, being eaten alive, or even being murdered by their own children. Others die in more everyday ways, like illness, car accidents, childbirth, and suicide.
Despite the frequency with which the trope appears, it serves as a catalyst for compelling character development on the part of their sad orphaned children, and can help darken the mood of a particular series. Vote up the characters whose loss affected you the most, and left you wishing you were watching something happier, like Digimon. Oh, wait, Iori's dad dies in Digimon. Never mind.