Rinnegan

The Rinnegan (輪廻眼; Literally meaning "Samsara Eye") is said to be the most revered and powerful of all the three great doujutsu. The Rinnegan's occurrence is so rare that once believed its existence was only a legend, and Kakashi Hatake believes it to be only a mutation. There have thus far been only two known users: the Rikudou sennin and Nagato. With the Sage long deceased, and Nagato now dead there are no other confirmed Rinnegan users alive at this point. The Rinnegan is characterized by a ripple-like pattern around the pupil, with a grayish-purple iris and sclera. In addition to enabling the user to quickly master various jutsu, the user may also use all six forms of chakra nature transformation which, as noted by Ibiki Morino, literally allows its user to use any technique they wish. It also allows the user to use all of the six paths techniques, a series of ninjutsu unique to the Rinnegan user. Like the Sharingan and the Byakugan, the Rinnegan also allows the user to see chakra, including otherwise hidden barriers. According to Nagato's partner Konan, the possessor of the Rinnegan can also control the seventh path of Pain, who is "beyond life and death", that can use the Gedo Rinne Tensei, which allows the Rinnegan user to revive all people who have recently died. It is later revealed that Madara has become interested in obtaining the Rinnegan to prepare for war, and for his own ambitions.
 * Because Nagato's Six paths of pain were all reanimated corpses, their Rinnegan was added to their bodies by Nagato through some unknown means. By doing so, Nagato could see each individual body's field of vision, coordinating the bodies with extremely precise planning, even during battles. Due to this effect, unlike the Byakugan, the Rinnegan does not have a blind spot, as long as the bodies are looking in every direction or at least one body is looking at the desired target. Because of this advantage, an opponent must blind one body, or distract all others in order for a blind spot to be opened. The animals summoned by the Chikushodo also possessed the Rinnegan, suggesting that Nagato controlled and could see through them as well. Madara uchiha also stated that while the Sharingan and more with the Mongekyou sharingan can translate some of the text on the Tablet the Sage left behind, only the Rinnegan can fully read the words on it.

Trivia
Rinne (輪廻) is the Japanese term for Samsara, the cycle of reincarnation or rebirth in several Indian religions. This is reflected in the names of Six paths of Pain, which share the names with Samsaric realms of reincarnation. It is unknown if the Rinnegan can be deactivated like the Byakugan and the Sharingan.
 * Contrast to the anime depiction of it being multi-colored, the manga has shown that the Rinnengan actually is grayish purple in color. This mistake was most likely done due to the manga not releasing the color in time for the anime. This error was corrected as of episode 80 of Shippūden.