As discovered in Attack on Titan, The Walls themselves are stuffed with dormant Colossal Type Titans. How they got there and why is still a mystery.
Mag Mel/Emperor Barodius and Razenoid/Dharak started the fourth season of Bakugan as this. Credit must be given to Code Eve for being extremely thorough though. He was sealed inside armor crafted from his own evil, then imprisoned in an alternate universe, and tied to his own throne with powerful webbing. The only reason he was able to free himself was simply because he had a Psychic Link to Dan and Drago, allowing himself to become powerful enough to free himself, something the one who put him there wasn't aware of.
Central 46 decide the only way to handle Aizen is to imprison him in their most secure prison. For 20,000 years. His fusion with the Hougyoku cannot be undone and renders him almost unkillable. As a result, the only option is imprisonment. In the Final Arc, the Bigger Bad mysteriously offers Aizen the opportunity to leave prison if he's willing to be recruited. Equally mysteriously, Aizen refuses, choosing to remain in prison.
The Quincies know of ancient folklore that speaks of a Sealed King. According to the legend, he would regain his heartbeat after 900 years, then his intellect after 90 years, followed by his power after 9 years. A secret final stanza adds that he will then regain the world after 9 days. Mysteriously, he wanted Aizen on his side for this. Equally mysteriously, Aizen refuses.
In Deadman Wonderland the "evil" are people affected by the "Branch of Sin" dangerous blood-manipulating powers who are secretly hidden deep in the "can", a maximum security prison/publicly open amusement park
In Digimon Frontier, Lucemon was sealed away in the Digital World's core. Some time later, he ends up using the Royal Knights to scan the entire Digital World and its three moons, effectively vaporizing them, to break free.
In Dragon Ball, the original Big Bad, Emperor Pilaf, uncorks Piccolo Daimao ("Piccolo, King Of The Demons"), the Sealed Evil In A Can who had previously been trapped in a Denshi Jar by Mutaito-sama centuries before the start of the series. (The Denshi Jar gets bonus points for strangeness; it is, in fact, an electric rice cooker with a holy seal glued to it.) Piccolo is revealed to be a Gnostic version of Satan, being the pure evil that the current God expelled to get "pure" enough to get his position in the first place (oh, and God is a Sufficiently Advanced Alien). The trope is somewhat subverted, however, in that, one Cloning Blues and one Heel-Face Turn later, Piccolo joins the good guys. And eventually even fuses with his "good" half when they desperately need a power boost.
Later, in Dragon Ball Z, the five-million-year-old sorcerer Babidi comes to Earth seeking energy to power Buu, the Sealed Evil In A Can that ate God's superiors. Originally, Toriyama wanted to finish the series after the defeat of Frieza, the ruler of the galaxy and the most powerful being in the universe. When the series' success made this impossible, he was forced to invent ever more implausible reasons why the ever more powerful villains hadn't shown up before and kicked Frieza's rear end. Androids, super-clones made from our heroes' DNA... by the time Babidi came to Earth, the current villain (Dabura) was the evil God of a Bizarro Universe. When he couldn't top that, Toriyama simply pulled Buu out of a Can.
Toriyama acknowledges his reliance on the Sealed Evil In A Can formula in the Neko Majin Z parody comic, in which the titular "hero" keeps a powerful demon sealed in a small rock balanced on top of a boulder by the side of a road. If anyone picks up the rock, the demon is released, requiring Neko Majin to step in and re-seal the demon... for a fee.
In the 13th movie, Wrath of the Dragon, the monster Hirudegarn is split in two and sealed inside Tapion and Minosha. Hoi breaks him free and rejoins the two halves... You guessed it, Hirudegarn stomps on Hoi and kills him. And Hirudegarn is eventually killed by Goku (who at that point used a technique that could have defeated him right from the start).
In the 9th movie, Bojack Unbound, the Galaxy Soldier, Bojack was sealed inside a star by the four Kais who would remain there. But due to North Kai's death by Goku's sacrifice with ready-to-explode Cell, Bojack was freed from imprisonment and heads towards Earth where Cell fought.
Liselotte Werckmeister in Eleven Eyes was sealed by the Black Knights in a Crystal as she is an immortal and intends to destroy the world. Though in the finale of the story, the seal is undone. In the anime only ending, once again she is sealed away to a spacetime rift with Kakeru and the black knights combine effort. In the original game novel, Kakeru manage to finally kill her off for good by destroying the Voidstone within Liselotte using the Eye of Aeon.
Zeref of Fairy Tail is revealed to be this right after someone wastes their life trying to raise him from the dead.
Even worse, the guy who wasted his life was just a diversion while someone else got the keys to unseal Zeref, who reveals later that he was never even sealed. He also isn't really that evil in the present.
The ghosts of Ghost Stories, until the construction sites destroy the places where they had been imprisoned. Satsuki then has to spend the rest of the series re-sealing them.
In the Gonzo version of Hellsing, the final episode reveals that Incognito plans to release the Egyptian god Set in order to destroy the world. Alucard defeats the released Set, it is left ambiguous if Set was really destroyed or just resealed.
Alucard himself used to be in a state of hibernation as a dead, rotting husk in a basement. And then someone found a way to splatter him with blood...
Inuyasha is a half-demon who was sealed to a tree by the priestess Kikyo's arrow for 50 years, until Kikyo's reincarnation Kagome freed him out of necessity. His apparent "evilness"didn't last long.
The seal is played straight with Ryuukossei, however. The powerful youkai that killed Inuyasha's father is first mentioned only when it becomes necessary for Inuyasha to succeed in an achievement that surpasses his father so he can master his sword. Cue introduction of Ryuukossei who had never been mentioned prior to that point. Reason being that Inuyasha's father managed to seal Ryuukossei with a fang the way Kikyou had sealed Inuyasha with an arrow. Naraku frees Ryuukossei who fights Inuyasha and dies, allowing Inuyasha to surpass his father and master his sword.
The Shikon no Tama. Unique in that it is both Sealed Evil In A Can andSealed Good in a Can. It contains the spirits of many demons that were once one entity and a powerful priestess locked in battle.
An anime-only example happens when the cast actually tries to seal away something evil to create this trope. When Toukijin's creator is killed as a result of the sword possessing him, the sword lands in the middle of a field. Even Ultimate Blacksmith Toutousai is unable to approach its immensely powerful evil aura, leaving them at a loss what to do. In the manga, this is the moment where Sesshoumaru (who commissioned the creation of the sword) arrives, defeating and mastering the sword as soon as he touches it. In an Adaptation Expansion, the anime, has the blade begin to corrupt the ground around it as soon as it lands, spreading swiftly. Since none of them can approach it, they debate what to do and Miroku decides to build a small shrine around the sword, blessing it to keep Toukijin's evil from spreading. Only then does the anime allow Sesshomaru to arrive to defeat and master the sword.
In the third movie Swords Of An Honorable Ruler, the evil sword Sou'unga was also sealed away for a few hundred years, only to break the seal while in Kagome's time and go about trying to kill everyone.
In the third part of Jojos Bizarre Adventure, legendary antagonist Dio Brando is released from imprisonment in a casket at the bottom of the sea by some unwitting salvagers, where he'd spent the last 100 years since the end of part one.
In Magical Girl Lyrical Nanoha A's, the Book of Darkness regenerates every ten years, sealing its programs (the Wolkenritter) inside of itself until then. Also, Gil Graham plots against both the heroes and Wolkenritter to allow for the Book, Wolkenritter and their innocent mistress to be sealed in the space between dimensions forever, because the Book would just keep regenerating. He is stopped and regrets ever deciding to put these lives at risk.
Mahou Sensei Negima! features two examples, one completely straight and the other a little bizarre. First, as the straight example we have Ryomen Sukuna no Kami, a gigantic four-armed two-faced demon god sealed in the Kansai region of Japan; releasing and controlling him was the motivation for the villains of the Kyoto arc. Second, and rather more unusual, we have the ancient and legendary vampire sorceress Evangeline A.K. McDowell. While she was a wanted magical criminal, responsible for multiple heinous crimes and with a huge bounty on her head, her sealing was based less on her evil and more on her obsession with the main character's father, who made sure she would NOT get out of a certain Boarding School... And, once you get to know her, she's not really as evil as she liked to claim... to the point that when the aforementioned demon god is released, the protagonists defeat it by breaking the seal on Evangeline and letting her deal with it.
Played straight again with the Lifemaker, the ultimate Big Bad of the setting, who is sealed inside the Sacred Tree of Mahoro Academy.
Tailed Beasts in Naruto are generally Sealed In Side A Person Shaped Can because it lets people use their power, there are a couple cases where they've been sealed inside objects: Shukaku was said to have been sealed in a teapot, the 8-tails was sealed in a giant jar (which can seal people in it as well), and after removing the Ten Tails chakra the Sage of the Six Paths sealed it in stone and put it in the sky, creating the moon.
After acquiring chakra material of all 9 Tailed Beasts, Tobi/Obito uses the Demonic Statue of the Outer Path as a vessel for its resurrection.
The manga has subverted this recently. It's been revealed that the tailed beasts aren't inherently evil, it's just that most of them hate humans due to (repeatedly) being used by them throughout history. Killer Bee, and later on Naruto, eventually get the beasts sealed in them to make a Heel-Face Turn by actually respecting them as sentient beings rather than treating them like mindless monsters.
It turns out the Ten-Tails wasn't evil at first either. It didn't even have a mind of its own since it was a tree. The Sage and his brother sealed their mother Kaguya inside the God Tree since her power had driven her mad. This only partly worked, since Kaguya fused with the God Tree and transformed it into the Ten-Tails to take her sons' chakra. That's when the Sage and his brother sealed the Ten-Tails, not realizing that their mother was the one controlling it. This is a weird case of one sealed evil in a can turning its can evil, making it necessary to seal the can too.
In One Piece, Impel Down, the world's most secure prison has a level that most people don't even know exists, Level 6 the "Eternal Hell" which contains at least two ex-Warlords of the Sea former Big Bad Sir Crocodile & Jinbe, Luffy's brother & Whitebeard Division leader Ace & Shiliew, the insane ex-Head Jailer who was a match even for Warden Magellan. And with the exception of Ace, all of these and more are free thanks to Luffy and Blackbeard.
Caribou, sealed evil in a barrel. Poor mermaids.
After "Him" in Powerpuff Girls Z is defeated horribly by a bunch of girls, he is contained in a tomb in an abandoned museum until the present day at which point he sends out a mummy to find the means to release him from his prison. His main goal is to conquer the world and destroy the Powerpuff Girls for their resemblance to the Ōedo Chakichaki Musume, the first three to defeat him.
The anime version of Prétear starts with the villain breaking free after sixteen years of being sealed away. And in the manga version, she remains sealed—but still manages to possess others anyway. Specially true in the case of Himeno's stepmom Natsue, her main "host".
In Princess Tutu the evil raven was sealed away by a prince who broke his heart into shards; the raven gradually gains power through the series and is fully freed when the prince's heart is returned to him.
Most of the villains in Sailor Moon were sealed until recently, except for the last season. In the first season, inside the can (the Dark Kingdom/Negaverse) was another can, with another, bigger Sealed Evil (Metallia/Negaforce) inside it; the forces of the Dark Kingdom spent most of their time draining energy from people in order to charge the metaphorical can opener needed to release her.
In the last season (in the anime at least), it is revealed that at the end of the first Sailor War, Sailor Galaxia had sealed Chaos inside her own body, and sent away her own Star Seed (which would become Chibi Chibi) in hopes to not have it corrupted.
Poseidon from Saint Seiya, sealed in a jar. Literally. He manages to employ Grand Theft Me on a boy named Julian Solo, and through Julian he gathers the Marine Generals in hopes of defeating Athena and her Saints so he can properly get out. He not only fails, but is again sealed.
Slayers has Ruby-Eye Shabranigdo, which was broken into seven pieces and sealed in humans (and gets out every now and then), Zanafar, and for good measure Shabranigdo's lieutenant Chaos Dragon Gaav.
In Soul Eater, the Sealed Evil in a Can is the Kishin, an insane creature who wreaked havoc upon the world until being sealed. The people working for the Shinigami do so to prevent the creation of another Kishin, which would occur if someone consumed a large number of human souls. It is not until half-way through the series that the main characters discover that the "original" Kishin is still alive, and imprisoned underneath their city.
Not only does said Evil get out, but the manga later reveals that there are similar creatures trapped elsewhere, although the Great Old One in Eibon's book has more in common with Shinigami than their estranged comrade Asura.
At the end of the manga, the Kishin is left as Sealed Evil on the Moon by Chrona. Hir method of doing so leaves the both of them, plus the moon, in a giant bubble of black blood.
The 3 big bad Tayutai in Tayutama are strong, but their method of sealing might have needed to be reinforced. They were freed by a runaway moped. On the other hand, it let Mashiro out into the world, so it all balances out.
Ryoko, from Tenchi Muyo!, was sealed away as a traditional demon in the OVAs; however, after calming down by dueling Tenchi for an episode, she joined the cast as a protagonist and suitor for Tenchi's hand. Washuu was in a similar situation in the TV series.
In Umineko no Naku Koro ni, Beatrice; The Golden Witch, The Endless Witch, The Witch Who Has Lived For A Thousand Years; is set up as this. However, the longer the story goes on, the less that seems to actually be the case. According to her the can she was sealed in was originally meant to seal something else. This is also most likely not the case.
In Yu-Gi-Oh!, Pharaoh Atem took the contingency plan of sealing both the Big Bad and himself 3000 years ago, until both resurfaced roughly simultaneously when their respective containers were found by archaeologists.