Face of Boe

The Face of Boe was a male-appearing Boekind whose outward appearance was little more than a gigantic, humanoid-like head. Although he would rarely do more than grunt vocally, he was able to communicate by means of telepathy. (DW: New Earth)

Early history
The Face of Boe had the distinction of being one of the oldest creatures in the known universe. He was supposedly 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, although Jack had done some time travel even before he met the Doctor. (NSA: The Stealers of Dreams) In 200,000, he was featured on Bad Wolf TV, at which point he had six living children at the time (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
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 and aftermath
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. This was the only time the Face of Boe communicated vocally with his only words out loud being "you are not alone." (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 memories also indicate that he originated in the Silver Devastation. (DW: Utopia).

When Davros asked the Doctor How many have died in your name?; The Face of Boe was among those who appeared in his flashback. (DW: Journey's End)

Though it is possible that the Face of Boe has permanently died and is indeed Jack Harkness, he is said to be a fact of the timeline and might exist in another form, or the "current" Jack, for a yet to be seen reason, will not become the Face of Boe.

Possible origins
After the death of The Master, 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 age into the being known as the Face of Boe (DW: Last of the Time Lords).

Although the Face of Boe appeared to have died permanently (DW: Gridlock) this does not disprove the theory that he was once Jack Harkness. The Doctor simply may not have stayed long enough to witness Boe's resurrection, which is possible given that he did not suspect a connection between his two friends at the time and the more life energy Jack expels during a death the longer it takes him to be revived (TW: End of Days). Also, Boe's death may have been a metaphor for a desire to end the current life he was leading. Jack has been shown to have lived several different lives, most of which were presumably ended by his many deaths (TW: Children of Earth: Day Five). Alternatively it may be that, rather than being truly immortal, Jack Harkness instead had an excess of life. This could explain why he constantly heals from death, but does eventually die.

Behind the scenes

 * In a feature for Doctor 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 following year, he realized after the fact that 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.
 * Despite the revelation in Last of the Time Lords, the actual origin of the Face of Boe is still debated, with Davies himself waffling during the commentary for the episode as to whether Jack really is destined to become the Face of Boe. Producer Julie Gardner, however, has been more assertive in this regard, most recently stating outright at the 2008 San Diego Comic Con that the Face of Boe is Jack Harkness. Similarly, the Torchwood Declassified featurette from Torchwood Series 2 has John Barrowman, David Tennant and Russell T Davies explicitly stating that the Face of Boe is Jack Harkness. In-universe, however, we only have Captain Jack's testimony.