Eighth Doctor/Other realities

There existed alternative versions of the Eighth Doctor in other realities, such as parallel universes and alternate timelines.

Aborted timelines
In a negated timeline, the Doctor and Charley were drawn towards a Vess weapons factory by a mysterious signal, where they also found the Fourth Doctor and Leela and discovered was plotting their undoing by removing their TARDIS from history. After Charley and Leela vanished, the Doctors were caught in the explosion of the Fifth Doctor's TARDIS, but were saved by the Sixth Doctor. Once a plan to stop the Master was conceived, the Eighth Doctor went to keep the Master distracted with the Fourth Doctor, and, once the Fifth Doctor had ensured that the TARDIS would not explode, joined his other seven incarnations in preparing to time ram the Master's TARDIS. However, rather than kill the Master, the First Doctor instead turned off the automatic distress actions, which had brought all of the Doctors to the pocket dimension and triggered the TARDIS' destruction, making it so none of that had happened. (AUDIO: The Light at the End)

Whilst attempting to rescue Andy Davidson from a crashing spaceship, the Doctor arrived just after Andy had already been rescued by Mr Bird. Unaware Andy was no longer aboard, the Doctor searched for him and was caught in the very collision he'd tried to save Andy from. Clinging to the console of his shattered TARDIS, he was left adrift in space in an air bubble the ship created and talked to the voice interface, in the forms of Liv and Helen, as his air slowly ran out. He finally used the console to phone Liv and Helen to say goodbye as he died. This fate was negated by Helen using Mr Bird's time machine to stop Andy leaving early with Mr Bird. (AUDIO: Get Andy)

When the Cybermen allied with Rassilon to take over history, (COMIC: Supremacy of the Cybermen) the Doctor and Josie found that the TARDIS had been covered in Cybermats, before being confronted by Cybermen who stated that they wished to "save" them. (COMIC: Prologue: the Eighth Doctor) This timeline was eventually unwritten by Rassilon and the Twelfth Doctor. (COMIC: Supremacy of the Cybermen)

Alternate timelines
In one alternative timeline, the Seventh Doctor was shot by Nazis in Germany in 1955, and had his body taken into custody. Absconding after his regeneration, the Eighth Doctor adopted the alias "Johann Schmidt" and later told Elizabeth Klein that he was a collector who had stolen the Doctor's body and gained possession of the TARDIS key. In this role, he was able to manipulate Klein's analysis of the TARDIS so that she would believe she had come up with the idea to go back in time and talk to the Doctor while he was still in Colditz, allowing his younger self to learn about how history would be changed and take action to correct it. (AUDIO: Colditz, Klein's Story)

Parallel universes
There were several alternate versions of the Eighth Doctor in the Multiverse who lived vastly defferent lives to the one he led. For instance there was: Doc Gallifrey, who was a gun using protector of a town called Vortex City; Joe Smith, who ran his own detective agency; a wizard named Quiquaequod; Theta Stigma, who was still close friends with the Rani and appeared to regularly go to her for psychiatric advice; an incredibly violent cyborg who had a sonic weapon for a right arm; and one where he was an anthropomorphic cartoon cat. (COMIC: The Glorious Dead)

Other realities
In the Obverse, there existed a version of the Eighth Doctor, called "Jonny" by his mermaid mother, Magda. (PROSE: )

A mirror image of the Eighth Doctor was created in Mirror Edinburgh when the Eighth Doctor gazed into one of the Silversmith's mirrors. (COMC: )