Final answer:
The Platyrrhini (New World monkeys) and the Cercopithecoidea (Old World monkeys) are two groups of monkey species noted for their flexible dietary habits, thriving in diverse environments.
Step-by-step explanation:
Two groups of monkey species that are famous for their flexible dietary niche are the Platyrrhini, commonly known as New World monkeys, and the Cercopithecoidea, specifically cheek pouch monkeys (Cercopithecinae) and leaf-eating monkeys (Colobinae), which are part of the Old World monkeys. New World monkeys, such as marmosets and spider monkeys, are especially known for their versatility in consuming various foods and their prehensile tails that help them navigate arboreal habitats. On the other hand, Old World monkeys like macaques and langurs showcase dietary flexibility by living in a variety of environments, including urban areas where they might consume human foods.