Defictionalization: Theakston's Christmas Ale. Theakston was already a real brewery, but they didn't make a specific Christmas beer - until NCIS fans started enquiring about it.
Hey, It's That Guy!: The main cast have appeared in a lot of other shows. Mark Harmon, Rocky Carroll and Lauren Holly all costarred on Chicago Hope. Harmon was a mainstay on St. Elsewhere. Carroll had his own sitcom and was the voice of Derek Maza/Talon on Gargoyles. Holly had starring roles in Dumb and Dumber and Picket Fences. Sean Murray was on Hocus Pocus and JAG; he played two different characters in JAG so if you count it as being in the same verse as NCIS it means he's played 3 different characters in the same universe. Michael Weatherly was on Dark Angel. More recently, the actress who played Agent Lee, Liza Lapira, has turned up in Dollhouse and Dexter. After Caitlin Todd died, she morphed into Maura Isles. Ducky co-starred in The Man from U.N.C.L.E. as Illya Kuryakin (or as Eric Ashley-Pitt from The Great Escape). NCIS's pilot episode can count as a Mythology Gag of sorts, since Ducky at times appears to be the team leader instead of Gibbs, mirroring his prominent role in UNCLE. See also Actor Allusion. Also, although not likely to be brought up in an American show, McCallum was the Steel half of Sapphire And Steel.
Stargate SG-1 fans will wonder about how the SGC intends to get Jonas Quinn off for serial murder in "Skeletons". They might be further confused by Martouf being a dirty Metro cop in "Lost And Found". Master Bra'tac apparently has a double life as elusive ultra-forger Khamal Khakani. Colonel Makepeace appears as an ex-Army colonel running a mercenary band that's hunting Mike Franks' daughter-in-law in a season 7 episode. Valla murders a man who has been killing off her dealers in season 7 episode "Borderland". The late Don S. Davis appeared as a government official in the second season and contributed to one of McGee's most awesome moments. And Jack O'Neill's clone was apparently adopted by a Marine and who was forced to hold a classroom hostage by Mexican drug runners. Stargate Atlantis fans will notice that Michael kidnapped Director Sheppard, and Elizabeth Weir is an overworked ME in "Identity Crisis" and recurs as Ducky's temporary replacement. For the Stargate Universe fans there's the deceased Gloria Rush as the now deceased Lara Macy and Tamara Johansen as Dana Arnett from the season 5 episode 'Leap of Faith'.
One episode had a random teenager played by Zac Efron, in all his feather-headed glory.
Zain Nadir was a Mossad agent who Ziva slept with.
Season 8 had a guest-appears of Alimi Ballard as Special Agent Levin, that is, David from NUMB3RS.
The teenager who holds his class hostage with a bomb in Season 3 is Mike Newton.
Peter Burke plays one of Gibb's old friends in "Capitol Offense." He didn't kill the first person but did help cover the crime up with another killing. This one is especially surreal for fans, since the character cheated on his wife, an act which is pretty much as likely to happen on White Collar as Mozzie making gift baskets for the suits.
River Song has apparently taken up black market dealings and flirting with Gibbs.
Gibb's father? John Walton. No wonder he's good with a shot.
Sean Murray, who plays McGee, is the real-life stepson of the show's creator Donald Bellisario.
McGee's sister is played by his real-life stepsister.
In one season 9 episode, Abby shows the others a locket containing a picture of her mother. The picture is of Pauley Perrette's real-life (deceased) mother, Donna. The lock of Abby's mother's hair inside the locket was also that of Pauley's mother.
The young Gibbs is played by Mark Harmon's son Sean.