Charvaka
Sometimes it is considered as a part of Hinduism (Hindu philosophy) as the word Hindu is an exonym and was a geographical identity in ancient times but most scholars reject it as a part of Hinduism since it's also called sanatana dharma and Charvaka rejects basic concepts of sanatana dharma like rta, the authority of sacred text like the Vedas (which is considered as an authority in Hindu philosophies). It also rejects the existence of an Atman or a Brahman (om).