Are there any Jewish sects which regard the Oral Law, Talmud and other interpretations of Tanach as unlawful addition to the religion? If yes who are they and what are their beliefs?
Currently, there exists a small Karaite community which claims to follow only the written law from teh tanach. On a website purporting to represent them, they give a history which lists other sects which, over history, have mirrored their beliefs (including "Sadducees, Boethusians, Ananites").
It is important to note two other points, though. One is the claim that, institutionally, Karaites and other seemingly strict-text groups have incorporated the practices, understandings and traditions of other groups. See here. The other comes only anecdotally from my conversations with Karaites over the years. They say that they DO have an oral law, but one that is local to each community. When the tanach text refers to unwritten laws of ritual slaughter, the Karaite wise man of each village institutes, based on his own understanding, what those laws are. There is no over-arching sectarian definition of the traditions and unwritten rules, but they exist, group by group.
The Rambam discusses this, and is very severe about someone who does so from his own ideas (in contrast to a child who was raised this way):
A person who does not acknowledge validity of the Oral Law is not the rebellious elder mentioned in the Torah. Instead, he is one of the heretics and he should be put to death by any person.
Since it has become known that such a person denies the Oral Law, he may be pushed into a pit and may not be helped out. He is like all the rest of the heretics who say that the Torah is not Divine in origin, those who inform on their fellow Jews, and the apostates. All of these are not considered as members of the Jewish people. There is no need for witnesses, a warning, or judges for them to be executed. Instead, whoever kills them performs a great mitzvah and removes an obstacle from people at large.
To whom does the above apply? To a person who denied the Oral Law consciously, according to his perception of things. He follows after his frivolous thoughts and his capricious heart and denies the Oral Law first, as did Tzadok and Beitus and those who erred in following them.
The children of these errant people and their grandchildren whose parents led them away and they were born among these Karaities and raised according to their conception, they are considered as a children captured and raised by them. Such a child may not be eager to follow the path of mitzvot, for it is as if he was compelled not to. Even if later, he hears that he is Jewish and saw Jews and their faith, he is still considered as one who was compelled against observance, for he was raised according to their mistaken path. This applies to those who we mentioned who follow the erroneous Karaite path of their ancestors. Therefore it is appropriate to motivate them to repent and draw them to the power of the Torah with words of peace.