Ultima 1: The First Age of Darkness (PC)

Movie (Star Wars): At one point during the game, you must go through a space-battle sequence. The enemy ships you shoot down here bear an astonishing resemblance to TIE Fighters. (Credit: Devin de Gruyl)
Status: Unverified

Ultima 2: The Revenge of the Enchantress (PC)

Movie (Time Bandits): One of the focal points of this game is mastering the use of the "Time Doors" that connect the different eras to one another, with the final battle taking place in the "Time of Legends." Ultima series creator Richard Garriott has stated he took this idea from the movie Time Bandits. (Credit: Devin de Gruyl)
Status: Unverified

Real Place (McDonald's): In 1990 AD, in the town of New San Antonio, you can buy food provisions from a "McDonnal's" restaurant, complete with drive-through window and a clown statue in front. (Credit: Devin de Gruyl)
Status: Unverified

Music (Sister Sledge): In some towns, you may encounter a priest-like NPC called Sister Sledge, who simply says "We are family!" That, of course, is the title of a 1978 song by the band Sister Sledge, perhaps best known as the anthem of the 1979 Pittsburgh Pirates en route to their World Series run that year. (Credit: Devin de Gruyl)
Status: Unverified

TV Shows (Looney Tunes): At one point you'll have to take off for a mysterious "Planet X" in order to complete the game. This has strong overtones of the classic Looney Tunes short "Duck Dodgers in the 24th-and-a-Half Century," which saw Daffy Duck doing comic battle with Marvin the Martian on Planet X. (Credit: Devin de Gruyl)
Status: Unverified

Ultima 6: The False Prophet (SNES/PC)

Music (Rule Britannia): The music in Lord British's castle is "Rule Britannia". The town of Lord British's castle is even called "Britannia". (Credit: ACC Kain & Flying Omelette)
Status: TRUE: Verified by FO.

Programmers: Lord British, himself, is based on Ultima's creator, Richard Garriott. (Credit: ACC Kain)
Status: TRUE: Verified by FO, source: Nintendo Power.

Literature (The Wizard of Oz): Lord British says that his favorite book is The Wizard of Oz, and you can find the book in the Lycaeum and bring it to him. (Credit: ACC Kain)
Status: TRUE: Verified by FO.

Ultima 7: The Black Gate (SNES/PC)

Cross-Series (Wing Commander 2): Cheaters out there, be sure to take note of the tune that plays in the secret solution rooms. If you've also played Wing Commander 2, you'll know where that them came from. It's the one that plays during cutscenes involving the Kilralthi. (Credit: Ultimate Chicken)
Status: Unverified

Cross-Series (Wing Commander 2): Another Wing Commander 2 cameo occurs when you talk to a farmer on the outskirts of Britain. He says he was assaulted by a group of cat-like humanoids going 'kill-wrathy' (if this doesn't already sound familiar, smack yourself on the forehead, please). The starship they landed on happens to still be on his field. Walk in the direction he points out. Eventually you will come across a craft sitting in the field that looks obviously Wing Commander Kilralthi-ish...which it is. Pity you can't enter it. This craft did appear in Wing Commander 2, but it wasn't listed in the manual. It is actually the "final boss" of that game, piloted by Prince Thrakath. Incidentally, the Kilralthi cutscene theme also plays in the ship's vicinity. (Credit: Ultimate Chicken)
Status: TRUE: (Screenshot) from MobyGames.com proves it.

Specific Reference: For a little reference to this game's predecessor (Ultima 6) go look for a lone horse in a barn, just behind Iolo's home (near Bee Cave), and talk to it. (Credit: Ultimate Chicken)
Status: Unverified

Mythology (Scientology): The focal point of this game is uncovering the truth about The Fellowship, a seemingly benign "self-help group" that is secretly plotting to overthrow Lord British and install the Guardian as the ruler of Britannia. The Fellowship recruits its members through a simple philosophy and outwardly harmless community service; it is not until members are well and truly indoctrinated that the truth is revealed to them - and by this time, their free will has been all but drained away. The Fellowship as a group is intended to be Richard Garriott's commentary on the so-called Church of Scientology, complete with an L. Ron Hubbard type in Batlin. Like The Fellowship, Scientology lures you in with a siren song of easy answers to all of life's pressing problems (in this case, through the book "Dianetics"), then gradually, through subtle means, siphons away your will to dispute anything you are told by fellow Scientologists (to say nothing of your bank account) until, finally, it is far too late to do anything but accept the utterly ridiculous "truth" of the religion as absolute gospel. (Credit: Devin de Gruyl)
Status: Unverified

TV Shows (Star Trek: TNG): The town/castle of Serpent's Hold is populated by characters deliberately evocative of the cast of Star Trek: The Next Generation. You've got the town's leader John-Paul, his right-hand man Richter, a captain of the guard named Horffe, Leigh the healer, and so on. Their personalities are all pretty much as you'd expect, too. (Credit: Devin de Gruyl)
Status: Unverified

Ultima 7 Part 2: Serpent's Isle (PC)

Programmers: If you take the subquest that requires you to kill Captain Stokes, be sure to have a look around his bungalow after the deed is done for a sight gag and some Origin plugs. For one thing: Check out the arrangement of the "Glowing Cube" and "Runic Box". Does this not look like some modern implement? (Credit: Ultimate Chicken)
Status: Unverified

Cross-Series (Various): Better yet, examine the "Glowing Cube" (don't touch the Runic Box; doing so will cause the contraption to blow up in your face. It causes no damage, but you'll miss out on the shameless plugs). You'll find ads for: