Spock

In a parallel universe, Spock was a Vulcan crewmember of the USS Enterprise in the 23rd century.

Biography
On one occasion, the Eleventh Doctor recalled memories of his fourth incarnation meeting up with James T. Kirk and the command crew of the USS Enterprise to combat a CyberNomad invasion of Aprilia III. With Montgomery Scott, Spock fired his phaser at the Cybermen, the beams distracting the invaders long enough for Kirk and the Doctor to defeat the Cybermen. Following the Eleventh Doctor and the Enterprise-D defeating the cyber-web, the Doctor suspected that his encounter with Kirk's crew had been erased from the timelines. (COMIC: Assimilation²) Spock was seemingly present at the Mars Old-Fashioned Alcohol Bar in the 26th century. (PROSE: Between the Wars: A Slow Night in Paradise)

Spock appeared to have also visited the Dalek Dome, alongside Captain Kirk. (COMIC: Liberation of the Daleks)

Other references
A fictional character named Spock appeared in the science fiction television series Star Trek in the Doctor's universe. When the Third Doctor told Sgt. John Benton that he had visited a parallel universe, Benton asked, "You mean like that Star Trek episode where Spock had a beard?" (PROSE: The Face of the Enemy) This character was played by Leonard Nimoy, and at least once said, "The needs of the many outweigh the needs of the few." (PROSE: Lonely) "Spock's Brain" was an episode of Star Trek which both Izzy Sinclair and Destrii had seen. Destrii even went so far as to call it her favourite. She cited "the bit where Sulu has to do the "captain's log" as a particularly memorable moment and even quoted back a portion of the log, in unison with Izzy: "Captain Kirk's hunch that Spock's brain is on this planet appears to be correct!" (COMIC: Ophidius)

Spock was a character that Rose Tyler associated with a more "professional" approach to the use of technology. Whilst in London in 1941, she once bemoaned the Ninth Doctor's reliance on the sonic screwdriver and conversation as his primary investigative tools. She demanded that he "give [her] some Spock" in their search for a downed Chula ambulance. Later, in an effort to explain to Captain Jack Harkness who the Doctor was, she gave the Time Lord the alias "Mr Spock," because she suddenly realised that she didn't even know his proper name. (TV: The Empty Child)

Donna Noble also seemed familiar with the character of Spock. She compared the Tenth Doctor's method of reading a person's thoughts to that of Spock's "mind meld". (AUDIO: The Nemonite Invasion)

After George Sheldrake's time tunnels had been stopped, the Tenth Doctor jokingly referred to Mark Seven as "Mr. Spock" to the confusion of his 41st century audience. (AUDIO: The Wrong Woman)

Earth-33⅓
Spock also existed in Earth-33⅓. When Star Trek 's Beverly Crusher won the Favourite Doctor Award at the Doctor Who Magazine Annual Poll, Spock told Captain Kirk that it was "'acting', Jim[,] but not as we know it". (COMIC: Doctor Who? 176)

Behind the scenes
The mentions of Mr Spock in The Face of the Enemy, Lonely, Ophidius, The Empty Child and The Nemonite Invasion treat him as a fictional character (as do, implicitly, other references to Star Trek and Star Trek: The Next Generation in Doctor Who stories). Assimilation² treats him as a "real" individual in an alternate dimension. This is similar to the treatment of Doctor Who in COMIC: TV Action! and COMIC: The Girl Who Loved Doctor Who.

Assimilation² depicts Spock with the likeness of actor Leonard Nimoy, who originally portrayed him in Star Trek. Leonard Nimoy was seriously considered to direct the 1996 TV movie, as recounted in the book, The Nth Doctor.

J. K. Woodward, who worked on the Star Trek crossover Assimilation², released pieces featuring Spock, gleaning the image of a jelly baby in a with the Fourth Doctor, assimilated by the Borg, in the midst of a cyber-conversion matching that of Craig Owens in Closing Time, facing a  with the Eleventh Doctor, and within the "coral" TARDIS control room.