1. Different versions of theme song from the '60s Iron Man cartoon can be heard throughout Iron Man. Once in the casino scene, once when Tony Stark and Christine Everhart get together, and again as Tony Stark's ringtone on Rhodey's phone.
2. The piano piece played by Obadiah Stane in one scene from Iron Man was composed by Antonio Salieri, who was infamously jealous of Wolfgang Amadeus Mozart's success, and may have even plotted his death.
3. The name of Iron Man's AI assistant, J.A.R.V.I.S., is an acronym for "Just A Rather Very Intelligent System."
4. A partially disassembled prototype of Captain America's shield can be seen in Tony Stark's workshop in Iron Man.
5. A clip of actor Bill Bixby, who played David Banner in The Incredible Hulk TV show from the '70s, can briefly be seen in 2008's The Incredible Hulk.
6. The voice of Bruce Banner from the '60s Hulk cartoon, Paul Soles, plays the owner of the pizza shop in The Incredible Hulk.
7. The pizza shop, Stanley's Pizza Parlor, is a reference to comic book icon Stan Lee.
8. Lou Ferrigno, who played Hulk in the The Incredible Hulk TV show from the '70s, makes a cameo as a security guard.
9. Composer Craig Armstrong also gave a nod to the original TV show when he echoed "The Lonely Man Theme" made popular by The Family Guy.
10. The news report from Culver University in The Incredible Hulk can also be seen on a screen at the end of Iron Man 2.
11. In one scene in Iron Man 2, Tony Stark is looking through a suitcase that belonged to his father, Howard Stark. Among his possessions are a Captain America comic book, and a map of the Arctic Circle, where Captain America's airplane went down.
12. A S.H.I.E.L.D. map seen at the end of Iron Man 2 indicates many points of interest for the group, including what's speculated to be Black Panther's homeland of Wakanda in Africa, Captain America's last-known whereabouts in the Arctic Circle, and Namor's undersea kingdom of Atlantis.
13. The comic book dragon creature, Fin Fang Foom, can be seen on a billboard for the Golden Dragon Casino in Iron Man 2.
14. Nick Fury tells Tony Stark that he has "bigger problems in the Southwest region to deal with" in Iron Man 2. He's likely referring to Thor's hammer being uncovered in New Mexico.
15. Several powerful Marvel comics relics can be seen in Odin's vault in Thor, including the Orb of Agamotto, the Tablet of Life and Time, the Warlock's Eye, and the Infinity Gauntlet.
16. The small New Mexico town in which Thor takes place is called Puente Antiguo, which translates to "ancient bridge" in Spanish. This likely refers to its connection to Asgard via the Bifrost Bridge.
17. A tourism poster in Puente Antiguo reads "Land of Enchantment… Journey Into Mystery." This is a reference to the comic book Journey Into Mystery in which Thor first appeared.
18. Writer of the Thor comic books, J. Michael Straczynski, makes a cameo as one of the men who attempt to pull Thor's hammer out of the ground in New Mexico.
19. When Natalie Portman's character, Dr. Jane Foster, hands Thor a t-shirt that belonged to an ex-boyfriend, a name tag that reads "Donald Blake M.D." can be seen on the shirt. Dr. Donald Blake was the name of Thor's alter-ego in the original comic books.
20. In the scene where Thor abducts Loki and confronts him in The Avengers, two ravens fly by while they argue. It's thought that these ravens are Huginn and Muninn, who serve as Odin's agents in Midgard in Norse mythology and in the Thor comic books.
21. The same ravens can be seen perched on either side of Odin's throne in Thor, and in Thor: The Dark World.
22. When Red Skull finds the Tesseract in Captain America: The First Avenger, he says that "the Fuhrer is too busy digging for trinkets in the desert." This line is a reference to the Indiana Jones movie, Raiders of the Lost Ark.
23. When we're introduced to Dr. Arnim Zola in Captain America: The First Avenger, we see his face distorted on a lens or screen of some sort. This is a reference to his character in the comic book, whose mind inhabits a robot, and whose face is displayed on a screen on the robot's torso.
24. While he's fleeing the Hydra fortress, Dr. Arnim Zola can be seen stuffing files into his briefcase. One of those files is a blueprint that appears to show plans for the robotic body the character inhabits in the comic books.
25. In The Avengers, actor Harry Dean Stanton, who plays the janitor who asks Bruce Banner. "Are you an alien?" is probably just nervous after his appearance in 1979's Alien.
26. In one scene in The Avengers, Tony Stark can be seen wearing a Black Sabbath t-shirt. This is likely a reference to the song "Iron Man," which isn't actually related to the Marvel character of the same name.
27. In all three Iron Man films, signs, buildings, and vehicles belonging to Roxxon Energy can be seen in several scenes. In Marvel comics, the Roxxon Oil Company is responsible for killing Tony Stark's parents.
28. During the New Years Eve scene at the beginning of Iron Man 3, Tony Stark drunkenly meets both Professor Yensin, who would later help him create the first Iron Man suit, as well as Dr. Chen Lu, also known as the super villain Radioactive Man.
29. The rock-man Thor battles at the start of Thor: The Dark World is Korg the Kronan, who appeared in the same comic book Thor first appeared in, Journey into Mystery #83.
30. In Thor: The Dark World, Dr. Selvig can be seen talking in front of a chalkboard covered in hidden references, including:
• The name "Simonson," which refers to Walt Simonson, the writer behind the Thor comic book for several years.
• A note about the 616 universe, which is the universe that most Marvel comic books take place in.
• A note about the Nexus of all Reality, which is a cross-dimensional pathway from Marvel comics.
• A note about the Fault, which in the Marvel comics is a rift in the universe created by Black Bolt.
• A note about the Crossroads, which is an alternate dimension in the Marvel comics that Hulk was once banished to.
• The names Kyle and Yost, which refers to Craig Kyle and Christopher Yost, who write for Marvel comics, and helped write and produce Thor: The Dark World.
31. The notebook Steve Rodgers writes in shows different important cultural touchstones depending on the country that Captain America: The Winter Soldier was shown in.
32. The S.H.I.E.L.D. freighter in Captain America: The Winter Soldier is called the "Lemurian Star," which is a reference to the sunken kingdom of Lemuria, which was home to the Deviants in the Marvel comics.
33. While revealing Hydra's secret plot to target threats in Captain America: The Winter Soldier, the double agent Jasper Sitwell mentions both Hulk's alter-ego, Bruce Banner, as well as Stephen Strange, also known as Doctor Strange.
34. The director of Captain America: The Winter Soldier, Joe Russo, makes a cameo as the doctor that patches up the Black Widow.
35. The writer of both the Winter Solider and Captain America comic books, Ed Brubaker, makes a cameo in the scene where the Winter Soldier gets his memories wiped.
36. In an interview, director Joe Russo joked that the driver of the Penske truck that allows Nick Fury to escape and that later hits and kills Agent Sitwell was possibly the Marvel anti-hero, The Punisher.
37. The character Brock Rumlow, who barely survives the crash of the S.H.I.E.L.D. helicarrier, can be seen with two straps across his chest as he's being resuscitated at the end of the film. This may be a reference to his character in the comic books, Crossbones.
38. Nick Fury's grave has the quote "The path of the righteous man…" on it from Ezekiel 25:17. This is a reference to the film Pulp Fiction. The character played by Samuel L. Jackson in that film has that same passage memorized, and recites it several times throughout the film.
39. The actress that plays Peter Quills mother in Guardians of the Galaxy, Laura Haddock, also appears in Captain America: The First Avenger. Her character asks Steve Rogers for an autograph.
40. The dog appearing in the hologram at the beginning of Guardians of the Galaxy belongs to the film's director, James Gunn. Its name is Dr. Von Spears.
41. A mural on the floor containing the Orb at the beginning of Guardians of the Galaxy depicts the four Cosmic Entities in the Marvel universe: Death, Eternity, Entropy, and Infinity. It also shows the six Soul Gems.
42. The alien Bereet who can be seen in Peter Quill's ship is wearing the same shirt that he was wearing when he was abducted as a child.
43. The actor who plays Yondu Udonta's second-in-command, Kraglin, is played by director James Gunn's brother, Sean Gunn. He also did all the motion capture acting for the character Rocket Raccoon.
44. Several artifacts can be seen in the background of the Broker's shop when Star-Lord tries to sell him the Orb, including what's thought to be the powerful Zodiac Key from the Marvel comic books.
45. Several important creatures can also be seen among the Collector's collection as well, including:
• A dark elf from Thor: The Dark World.
• A cocoon that may contain the Marvel hero Adam Warlock.
• A Chitauri soldier from The Avengers.
• Marvel character Howard the Duck, who also appears in a post-credit scene.
• A dog, presumably Cosmo from Marvel comics, who is said to have been launched into space by the Soviets, and who also appears in a post-credit scene.
• Slugs from Slither, which was also directed by James Gunn.
46. During his presentation to the Guardians of the Galaxy, the Collector also shows what appears to be Eson the Searcher, one of the powerful Celestials from the Marvel comic books, as well as a group that could possibly be the Watchers.
47. In The Avengers, Mark Ruffalo, as Dr. Bruce Banner, mentions a botched suicide attempt: "I put a bullet in my mouth… and the other guy spit it out." This suicide attempt is a deleted scene from 2008's The Incredible Hulk with Edward Norton as Bruce Banner.
48. In this same deleted scene, Captain America's iconic shield can briefly and barely be seen embedded in the ice.
49. Some of the Infinity Stones (or Soul Gems) that power the Infinity Gauntlet are thought to be seen throughout the Marvel films:
• The Tesseract in Captain America: The First Avenger - The Space Gem
• Loki's Scepter in The Avengers - The Mind Gem
• The Aether in Thor: The Dark World - The Reality Gem
• The Orb in Guardians of the Galaxy - The Power Stone
50. Stan Lee has infamously made a cameo in each Marvel film. Here are all his appearances:
• As Hugh Hefner in Iron Man.
• As the man who accidentally drinks Hulk's blood in The Incredible Hulk.
• As Larry King in Iron Man 2.
• As the pickup truck driver in Thor.
• As a military officer in Captain America: The First Avenger.
• As a man on the street interview in The Avengers.
• As a beauty contest judge in Iron Man 3.
• As a patient in the mental health facility in Thor: The Dark World.
• As a Smithsonian security guard in Captain America: The Winter Solider.
• As the "class-A prevert" Rocket Raccoon spots in Guardians of the Galaxy.