Tobi

Nothing much is known about him yet. The demon fox mentioned him to Sasuke Uchiha when he invaded Naruto's mind to supresses his chakra. The fox said that Sasuke's Sharingan and Madara's Sharingan had chakra more cursed than his own.

Later Tobi revealed himself as Madara Uchiha after surviving Deidara's suicide explosion. Tobi was seen giving Pein and Konan orders meaning that he is the true leader of the Akatsuki.

Speculations
Some people believe that Uchiha Madara is the third Uchiha clan member who has the ability processes the Mangekyo Sharingan. Some people also think that Uchiha Madara is the person on the other side of The valley of the end.