Dogon

The Dogon were a species known for their sixth eye.

Biology
The sixth eye was located on the back of the head and was nearly the size of a golf ball, though it looked more like a shifting blue marble. This eye allowed a person to look back clearly on their life and make better choices, especially if swallowed.

Oddly enough, Eugene Jones steadily gained more physical contact with the world after swallowing a sixth eye and then dying. He was able to influence the mind of Gwen Cooper and eventually became corporeal and seemed to float into an afterlife. (TV: Random Shoes)

History
In the 1980s, Mr Garret found a Dogon sixth eye while golfing. In 1992, he gave it to Eugene Jones as a consolation prize for a failure at a maths competition.

In 2007, Eugene decided to sell his Dogon eye on eBay. His friends Gary and Josh bid on it to cheer him up. However they accidentally won. They planned to buy the eye and resell it at a higher price. When they tried to take the eye by force, Eugene swallowed it and ran away. Though he was later killed, the eye brought him back as a ghost-like being, allowing him to help Gwen Cooper investigate his death. He remained halfway between life and death even when the eye was removed from him. Torchwood Three retrieved the eye. (TV: Random Shoes)

The collector Henry Parker also had a Dogon eye. (TV: A Day in the Death)