Alternate RealitiesThanks to a variety of incidents involving time travel and physics experiments gone wrong, we've met or heard of several Rodney McKay's from the alternate realities that exist alongside our own.
SG-1 8x19 & 8x20 - Moebius, Parts I & IIUsing a Puddle Jumper which had been equipped with a time travel device, SG-1 travelled back to 3000 BC and altered the timeline. In this new reality, Rodney was based at the Cheyenne Mountain complex and was the lead scientist working on locating the Antarctic Stargate. He and Carter clashed over the project and she was ultimately proved right. Rodney later declared himself glad not to be with the team who travelled through the Stargate. This reality's Rodney, like our own, had a crush on Carter, enjoyed wearing 'humourous' t-shirts and thought that Puddle Jumpers should be called Gateships. However, he was not allergic to lemon and stated that lemon chicken was his favourite food.
SGA 1x15 - Before I SleepOriginally, Atlantis was not designed to rise to the surface upon catastrophic failure of the shields. In this reality, the expedition was forced to evacuate the city almost immediately after arriving. Rodney remained in the control room as it filled with water trying to open the Puddle Jumper bay doors to allow those on the Puddle Jumpers to escape. He drowned when the control room was completely flooded.
SGA 3x08 - McKay and Mrs. MillerAfter a bridge between realities starts to cause dangerous build up of exotic particles in an alterate Pegasus galaxy, Rod McKay was dispatched to our reality to ask that the experiment be shut down. Rod was close to his sister and godfather to all three of her children. He seemed to enjoy taking risks and being adventurous. An early riser, he enjoyed golf, was modest about his intellectual gifts and was always eager to help other people (a trait which could become annoying as he repeatedly corrected Teyla on her Athosian history). Despite thinking his reality's Sheppard was a bit of a nerd, and his Teyla was difficult to talk to, Rod was eager to return to them because they were his team.
SG-1 10x13 - The Road Not TakenIn a reality where the Stargate programme was forced to go public, Rodney is a dot come millionaire who runs his own business, buying and selling companies, having left theoretical physics behind at some point. He was married to Major Samantha Carter but they later divorced. When approached by the Carter from our reality, he agreed to help her but before he could do much he was given the post of Special Advisor to the President at the SGC, seemingly against his will. This reality's Rodney wore glasses and seemed to still be in love with Carter, despite their divorce.
SGA 4x20 - The Last ManThe presumed death of Sheppard and subsequent failure to rescue Teyla from Michael sent an alternate Pegasus galaxy spiralling out of control. In this reality, Rodney was one of a small team responsible for preparing the Phoenix, a Daedalus class ship, for battle against the Wraith though he remained on Atlantis when the ship was finally dispatched. He later left the city with Keller when the IOA isolated Atlantis from the rest of Pegasus. During the trip home, Rodney and Keller became romantically involved and remained together until her death from complications caused by the Hoffan drug. On Earth, Rodney had worked for an Aerospace engineering firm, but he abandoned this to search for a way to change the timeline to prevent Sheppard's death which he believed to be the starting point for everything that had gone wrong. It took 25 years but, together with his sister, Rodney was successful, creating a hologram of himself that could help Sheppard get back to Atlantis and save Teyla.
SGA 5x04 - The Daedalus VariationsOur Rodney hypothesised that the Alternate Reality Drive responsible for pushing the Daedalus through multiple universes, was designed by an alternate Rodney. This is not certain and nothing is known about this other Rodney.
SGA 5x04 - The Daedalus VariationsOne of the alternate versions of Rodney encountered as the Daedalus skipped through various universes, starved to death after boarding the Daedalus with Sheppard, Teyla and Ronon. A data pad which confirmed that this Rodney had not bybassed the navigational system was found, but he had ultimately been unable to do anything to fix the malfunctioning Alternate Reality Drive.
SGA 5x19 - VegasIn this reality, Rodney was a member of the Atlantis expedition and of a Gate team but one of the primary differences between his reality and ours was that Sheppard had never travelled to the city, having been discharged from the Air Force after Afghanistan. Rodney was in Area 51 tracking a lone Wraith in Las Vegas (whether he was recalled from Atlantis for this was unclear). Having, at some point, met an alternate version of Sheppard he decides to trust his reality's Sheppard with the truth about Pegasus and the Wraith. When Sheppard destroys the Wraith's caravan and the signal being sent from it, Rodney realises that it contained the location of Earth but that Sheppard has prevented it from reaching anyone in their reality. This reality's Rodney wore a wedding ring and dressed quite formally.