Face of Boe

The Face of Boe was a male Boekind whose outward appearance was little more than a gigantic, humanoid-like head. Although he could only grunt vocally, he was able to communicate telepathically. (DW: New Earth)

Earlier history
The Face of Boe had the distinction of being one of the oldest creatures in the known universe. This caused him to be dubbed on some worlds as "the creature that God forgot". He was well known by the 51st century, since Jack Harkness commented on the fact that he once saw someone dressed up as the Face of Boe. (NDA: The Stealers of Dreams) In 200,000, he was featured on BAD WOLFTV, at which point he was pregnant with six children (Boemina) who died forty years later. (DW: The Long Game) In 200,100, he was the focus of a question on The Weakest Link, where he was described as the oldest living creature in the Isop galaxy. (DW: Bad Wolf)

Later history
"Legend says the Face of Boe has lived for billions of years. Isn't that right? And you're not about to give up now."

- The Doctor.

In the year 5.5/Apple/26, the Face of Boe was the sponsor of an event to safely witness the destruction of the Earth by the expansion of the Sun. The event was sabotaged by Lady Cassandra, but the Face was among the survivors. (DW: The End of the World) By 5,000,000,023, the Face was hospitalised in Ward 26 of the hospital run by the Sisters of Plenitude on New Earth. Apparently dying of old age, the Face summoned the Doctor to his ward. When a novice told the Doctor the legend of the Face's last words - a secret which the Face would impart only to one like himself - the Doctor realised that he fit the description of the "wandering traveller". (DW: New Earth)

Death of the Face of Boe and aftermath
"But know this, Time Lord. You are not alone."

- The Face of Boe.

The Face of Boe eventually recovered, saying that although he had grown tired of the universe, the Doctor had shown him a new way of looking at things. The Doctor asked about the message, but the Face told him that it could wait for their third and final meeting, and teleported away. (DW: New Earth)

The Doctor was ultimately reunited with the Face of Boe one last time during his travels with Martha Jones, in the year 5,000,000,053. Boe revealed his last secret with his dying breath after sacrificing himself to save the city of New New York: "you [the Doctor] are not alone". The Doctor dismissed this, as he still firmly believed himself the last Time Lord. (DW: Gridlock)

When the Doctor later met Professor Yana, the Master transformed into a Human, the Doctor realized the significance of the name "Yana", which stood for "You Are Not Alone". Yana's (false) memories also indicate that he originated in the Silver Devastation. (DW: Utopia).

Possible origin of the Face of Boe
After the death of Mr. Saxon, the immortal Jack Harkness said he lived on the Boeshane Peninsula, where, as the first Time Agent from that place, he was a poster boy and nicknamed "The Face of Boe". Given that the Doctor had just speculated that Jack might mutate in shape rather than age in the traditional way, this implied that he (Jack) might one day evolve into the being known as the Face of Boe.

Story Notes
In an feature for Dr Who Magazine, Russell T. Davies wrote of a prophecy where the death of the Face of Boe would be heralded by "the heavens opening". When he wrote Gridlock the next year, he realized after the fact that indeed the Face of Boe's death did indeed cause the heavens to open - his last bit of energy opened the sealed system to free the drivers of New Earth.