If you were a kid in the early 1990s, you were probably enthralled by X-Men: The Animated Series. While the cartoon doesn't hold up that well in the modern day, it was one of the first attempts to really bring the Marvel Universe to life on television and really paved the way for later, more ambitious attempts like the Marvel Cinematic Universe. Although most of episodes obviously focused on the X-Men, the show also had countless cameos from tons of recognizable Marvel characters. Most were just brief cameos, while other characters actually played a part in some episodes. Here's a list of every non-mutant superhero cameo we know of in X-Men: The Animated Series:
Alpha Flight, the Canadian superteam consisting of Vindicator, Shaman, Puck, Snowbird, Northstar, Aurora, and Sasquatch appeared in the episode "Repo Man" to try to convince Wolverine to defect back to Canada. Alpha Flight claimed that the Canadian government owned Wolverine's adamantium skeleton and demanded that he return so Department H could conduct additional tests. After the X-Men fought Alpha Flight off, Wolverine warned them never to return. Alpha Flight was later shown in a much more positive setting when they rescued civilians during "Phoenix Saga" from various meterological disasters. Northstar also appeared in the episode "Slave Island" as a prisoner of Genosha.
An alternate universe version of Nick Fury makes a cameo appearance in the episode "Time Fugitives" listening to a speech about the Legacy Virus while flying a SHIELD helicarrier. Fury also appeared as a member of the Howling Commandos (along with Dum Dum Dugan) in "Old Soldiers" fighting alongside Captain America and Logan. Fury made frequent appearances as the leader of SHIELD in the Iron Man and Spider-Man television series, but SHIELD was never an active presence on the X-Men cartoon so he never got a chance to shine despite his history with Wolverine.
War Machine makes two cameo appearances in X-Men: The Animated Series. War Machine and several other superheroes are shown saving civilians as the M'Krann Crystal threatens to destroy Earth in the show's version of the Phoenix Saga. He also appeared in an alternate universe timeline working with SHIELD in "Time Fugitives". War Machine played a much larger role in Iron Man: The Animated Series as Tony Stark's best friend. Interestingly, War Machine also teamed up with Spider-Man to stop a really odd team of Dormammu, Baron Mordu, Venom and Carnage during a multi-part storyline.
Kazar and his sabertoothed tiger Zabu made several appearances on X-Men: The Animated Series. He helped Wolverine rescue the X-Men from the Mutates and Mister Sinister's men in "Reunion" and reappeared in "Savage Land, Savage Hearts" when the X-Men returned to the Savage Land to rescue Storm from Sauron. His wife Shanna also made a cameo in "Savage Land, Savage Hearts" as one of Sauron's other prisoners.
Captain America appeared in "Old Soldiers" as Wolverine remembered a time during World War II when he fought alongside the Star-Spangled Avenger against the Red Skull. An alternate universe version of Captain America led the Avengers against the X-Men in an world where Xavier was murdered in the 1950s. The latter storyline was loosely based on the "Age of Apocalypse" storyline. It was also mentioned in "Red Dawn" that Omega Red was the Soviet's attempt to recreate the Super-Soldier formula. In the animated series timeline, Captain America wasn't frozen in ice, but was trapped with the Red Skull in a time vortex for 50 years. When the pair were freed in the modern day, Captain America sacrificed himself a second time to imprison the Red Skull back in the time vortex. He later escaped to join the Avengers during their own short-lived cartoon series.
Spider-Man was the star of Marvel's other big animated show of the 1990s, which ran even longer than the X-Men cartoon. He made a brief cameo in "Phoenix Saga" as one of the superheroes rescuing civilians as Earth experienced wild weather phenomena because of the M'Krann Crystal. The X-Men also made several crossovers into Spider-Man's own animated cartoon, where they helped Spider-Man deal with his slow transformation into the Man-Spider.
Doctor Strange made a couple of cameos during the Phoenix Saga, first when he notices the Phoenix suddenly drags the X-Men into space, and then later when Jean/the Phoenix leaves Earth to destroy a solar system. Basically, whenever the Phoenix Force showed up, Strange appeared in a reaction shot montage. Strange and his girlfriend Clea also showed up at a ski resort that the X-Men were staying at in "Nightcrawler" (along with Mary Jane Watson), although the X-Men didn't ask for his aid when dealing with a supposed demon (actually Nightcrawler) who lived in a nearby town.
Thor also appears in the montage of reactions in "Phoenix Saga" when Jean leaves to eat a solar system. Mjolnir reacts strongly to the Phoenix Force's planet, as if it knows she's about to wreck a galaxy far, far away. Thor played a much larger role in two Fantastic Four storyline involving Galactus, so of course he would have an interest with celestial beings with an insatiable hunger.
The Black Panther makes a VERY brief cameo in "Sanctuary" when Magneto takes several ships full of mutants up to Asteroid M in space. He's there for one second, and you only see him from behind. It's possibly the most pointless cameo ever. The Black Panther also appeared in the Fantastic Four and Avengers cartoons, where he at least got a few lines of dialogue.
Uatu is another one of the cosmically inclined beings who reacts when the Phoenix leave Earth. Uatu didn't bother to warn the X-Men about what was about to happen with Jean, even though he took the time to warn Earth about the coming of Galactus in the Fantastic Four cartoon. The Watcher is very inconsistent with his cosmic warnings, I guess.
Ghost Rider made a surprise appearance in X-Men: The Animated Series when he appears in one of Gambit's memories as Xavier telepathically probes his memories looking for references to Henry Gyrich. His cameo was a reference to a 1992 comic in which the X-Men help Gambit clear the Assassin's Guild of a Brood infection. Ghost Rider also had a major role in the Hulk animated series (he tracked the Hulk down for his assorted crimes) and also gave Galactus his Penance Stare, finishing the World Eater off in an episode of the Fantastic Four cartoon.
X-Men: The Animated Series had an entire episode focusing on Carol Danvers, who was then known as Ms. Marvel. As in the comics, Rogue accidentally stole Ms. Marvel's powers and left her in a coma, although Ms. Marvel's personality lived on in Rogue's mind. In order to keep the two psyches from battling for control, Xavier wiped Rogue's memories of the accident when she first asked to join the X-Men. Mystique later triggered Rogue's memories of the events in an attempt to force her to rejoin the Brotherhood, but all it resulted was waking up Ms. Marvel's psyche and causing another battle for control of Rogue's body. With the help of Jean Grey, Rogue retakes control of her body and later accidentally transfers Ms. Marvel's personality back into her body upon discovering the hospital that Carol was kept in.
A robot version of Punisher appeared in the episode "Mojovision" as one of the robots sent to attack Wolverine and Jean Grey. He also made a cameo on a video game cover in "Days of Future Past". The Punisher video game actually triggered Bishop's memories of the future, and led him to track down the person who was supposed to assassinate Xavier in the present day. Since these were children's cartoons, the Punisher never killed anyone in his appearances in the Spider-Man and X-Men cartoons, although it was implied that he was more violent than the average superhero.
The Hulk starred in his very own 90s cartoon, which never had a proper crossover with X-Men: The Animated Series. However, the Hulk did make a cameo as a Danger Room robot sent by Xavier to distract the Juggernaut when his evil stepbrother attacked the X-Mansion. Although it was a pretty cool moment, the Juggernaut wrecked the robot since the Hulk wasn't the real thing.
In the X-Men: The Animated Series version of the "Age of Apocalypse" timeline, the Avengers are shown as the primary antagonists hunting down mutants on behalf of the human race. In addition to Captain America and Iron Man, the evil Avenger roster also includes the Wasp, Giant Man, Hercules, Black Widow, Daredevil and the Scarlet Spider. This timeline was erased when Bishop, Wolverine and Storm successfully rescued Xavier in the past (before he was murdered by time travelers), although a few of these Avengers made other appearances in other 90s cartoons.