Many, many examples in Buffy (the Judge, Acathla, the Seal of Danzalthar, among others) and Angel (Illyria [who is actually sealed in a coffin], Pavayne,Sahjhan). In fact, the Angel example showed us a veritable warehouse of sealed demon gods like Illyria, which (as of the end of the series) is left unguarded.
There is also a very literal variant in the Buffy episode "Get It Done", when The Shadow Men try to infuse Buffy with the essence of a demon that they kept sealed in a box.
In the original and revived Dark Shadows, vampire Barnabas Collins made his series debut when he was released from the chained-up coffin in which his father, unable to go through with staking his own son, had sealed him.
Doctor Who: Around half a dozen cases of Evil deliberately Sealed in a Can. At least as many slumbering alien menaces that just happened to crash-land/get trapped centuries ago and go into hibernation until foolishly awoken, which don't technically fit the description but serve a very similar plot purpose.
In "The Satan Pit", a being known only as "the Beast" (who claims to be Satan) is so powerful it was sealed miles underground the surface of a planet precariously orbiting a black hole, meaning that any attempt to escape would send the Beast and the planet to fall into it. These Disciples of the Light guys really didn't want this thing to escape.
A slight variation happened in "The Stones of Blood", where the Doctor releases a pair of biomechanical judges called the Megara from a ship stranded in hyperspace, and the judges promptly sentence him to death for letting them free without the proper legal authorization.
In the new series episode "The Runaway Bride", it is revealed that the centre of the Earth contains hundreds — possibly thousands — of omnivorous intelligent alien spiders. Indeed, the vessel that holds them is the original core of the planet, nearly five billion years old.
In the episode "The Pandorica Opens", the Doctor investigates the eponymous Pandorica, which is advertised as containing the most feared being in the universe and is starting to open. It's empty. The Pandorica was created by a coalition of the Doctor's enemies to trap him inside, because they believe he will destroy the universe, and it's opening so that they can put him in — due to time-travel, he has arrived before the myth started. The Doctor is predictably fascinated; "What could be so dangerous?" he wonders. You.
Then there is the Old God in a Pyramid in "The Rings of Akhaten". It turns out the real Monster is the planet.
In Forever Knight, LaCroix's daughter was sealed inside a coffin after LaCroix staked her. The Egyptian sun disk kept her prisoner much like a Christian cross, until she was released by grave robbers and came after him and his children.
Also pops up in the first Forever Knight novel, "A Stirring of Dust".
In Fringe, the pieces of the First People's Machine, which has the power to create or destroy universes, were scattered all across the globe and buried hundreds of feet underground.
In Season 3, in a slight subversion, daddy Petrelli appeared to be far more powerful than Adam and took his power, killing him instantly.
Arthur Petrelli himself has elements of this, starting off the series on life support and only able to communicate through telepathy.
As of Volume 4's finale Sylar's brainwashing into believing himself to be Nathan Petrelli certainly qualifies. It didn't last.
Volume 5 has Sylar being imprisoned inside his own head and Parkman sealing his comatose body inside a brick cell.
One episode of Highlander: The Series featured a Nazi Immortal who had been chained and thrown into a river by a young boy. How do you make a Nazi cross? Forty years of drowning and reviving every few minutes ought to do it...
There is at least one story of an Immortal being on a sunken ship and having to walk back — Kit O'Brady was on a boat to Alaska when it sank, and said it took him months to walk back.
And then there's the example of the Immortal stranded on a deserted island for years, dying of starvation and thirst every so often, just to wake up and do it again. For some reason he held a grudge...
In Kamen Rider Kuuga, the Grongi were imprisoned in a tomb by the original Kuuga after he defeated them.
In Kamen Rider Agito, the Overlord of Darkness has a somewhat odd version of this, he was already dead, but left his DNA around, allowing him to retake his physical form when humans uncover it
Kamen Rider Blade has the Undead, who have been imprisoned since the beginning of time, after serving their purpose and deciding the dominate species of life on Earth. Though since not all of them are evil, some would be Sealed Good in a Can and since most of them just want to fight one another, they would be Sealed Badass in a Can. The Roaches and Black Joker defiently fit the trope though. There's also Jashin 14 in The Movie.
In the newest series, Kamen Rider OOO, the Greeed have been imprisoned in a stone box for 800 years until they're released during a robbery. The stone box? The original OOO.
In Lexx, Kia is an assassin who spends most of his time frozen so he's save his protoblood for when he's needed, which is only when someone needs to be killed.
One episode from Los Luchadores dealt with a plot to awaken a demon in a ritual involving three descendants of the people responsible for originally sealing it. In this case, the ritual is successfully interrupted just before its completion, denying the audience the spectacle of watching an ancient evil being defeated by a masked professional wrestler.
In LOST, Jacob claims that this is the role of the Island. It is implied that the Man in Black/the Smoke Monster is the evil being imprisoned.
Merlin: The goblin from '' Goblin's Gold'', who was sealed in a box, then escaped to torment everyone in the castle, and then resealed after being trapped by Merlin and Gwen.
In MythQuest, Gorgos was shut away inside a stone for trying to change the myths of humanity. Matt Bellows accidentally releases him in the first episode, but only into a slightly bigger can. With significant air holes.
A constantly recurring trope in Power Rangers, going all the way back to the first episode:
The Trope Namer Rita Repulsa from the first season of Mighty Morphin' Power Rangers was originally trapped in a "space dumpster" on the moon (or a moon, or something) for 10,000 years until astronauts accidentally released her. Her line from the intro sequence is rather iconic:
Ahh! After 10,000 years I'm free! It's time to conquer Earth!
Her boss Lord Zedd eventually returns, and displeased with her failure puts her back in her dumpster (or a different one) and sends her off into space again. She gets out twice. The first time the Rangers catch her and reseal her, the second time she stays out for good.
To a lesser extent, in the episode "Final Face-Off," Rita opens an urn which imprisoned the legendary Face Stealer. The Rangers lock him back in the urn upon his defeat at the end of the episode.
Power Rangers Lightspeed Rescue's villains are released from the "Tomb of Forever" by an unwitting Egyptologist. The villains then go about trying to free their still-sealed Queen. They all more or less end up that way again in the finale when they are either destroyed (sending their spirits back) or by physically being thrown back into the "Tomb of Forever", sealing them in the Shadow World.
While not applying to the main villains of Power Rangers Time Force, the individual monsters fit this trope, being mutant prisoners shrunk, cryogenically frozen, and sealed inside small containers. They also return that way when defeated.
The Orgs of Power Rangers Wild Force were spirits sealed in the earth 3,000 years ago. They were freed in the present day to possess everyday objects due to pollution and the appearance of a successor to their Big Bad.
Power Rangers Ninja Storm has the "Abyss of Evil" that acts as one. Unusually, the Big Bad, Lothor, didn't bust out of there at the beginning of the season; instead it factored into his Evil Plan: if his monsters couldn't defeat the Rangers, they'd be sent to the Abyss; cramming it full until it burst open and gave him an army to work with.
Double subverted, as Lothor is sealed in the Abyss at the end of the series, and then breaks out in the next season, Power Rangers Dino Thunder, during the team-up episode.
In Power Rangers S.P.D. most of the individual monsters and main villains fit this trope, being alien criminals. They are trapped inside special cards when defeated.
Rita Repulsa's very line is echoed by Koragg's first line in Power Rangers Mystic Force (He actually says "After all these years, I'm free," but the similarity was close enough that many fans were confused into thinking the backstory was set ten thousand years previous, when it actually appears to have been more like 20).
In Power Rangers Operation Overdrive all four big bad factions has someone sealed inside a different can. Moltor and Flurious were sealed inside two lifeless planets, one of extreme heat and the other of extreme cold respectively, when they tried to grab the season's MacGuffin, and freed when said MacGuffin was discovered on Earth. Kamdor is initially sealed in a blue gem necklace until he is freed with a stolen device by his partner Miratrix, who incidentally later becomes sealed in same said necklace when Kamdor double crosses her. Then there is the "Prison Mirror" which contains an army of Fearcats, but was destroyed with only Benglo escaping. In another episode, Thrax, the son of Rita and Zedd, repeats the same line his mother made when released from a Space Dumpster, after what could not have been more than two or three years.
Power Rangers Jungle Fury has Dai Shi, who has been stuck in an (unlocked!) box for 10,000 years. The box was kept by a secret order, but opened when a disgruntled student barged in and attacked the order's master, knocking the box out of his hands. Unusually, however, Dai Shi is significantly weaker than he was, and has to struggle to regain his powers before the Rangers become strong enough to destroy him.
This season also had other sealed cans. The Crystal Eyes for the Phantom Beasts, The Overlords lifeless bodies/shells & two of the "5 Fingers of Poison" urns with broken remains (both revived by Naja's life talons), the various statue soldiers brought to life as monsters of the day, and the Spirit World (at the finale) which acted as both a sealed can of evil (for Dai Shi's fallen minions) and a sealed can of good (for the fallen Masters).
Venjix from Power Rangers RPM began its existence more or less as a sealed evil. The program for the sentient, self-replicating, computer virus was initially created and contained in a single laptop. That is until Dr. K, in a attempt to escape her circumstances, decided to infect Alphabet Soup's computer network with the virus but was stopped short from installing a firewall to prevent further spread. The result was an infection of nearly every computer system connected to the internet and the destruction of nearly the entire human race.
It is hinted that Venjix also becomes one of these again, as opposed to being destroyed, at the end of RPM with the Venjix Palace music playing as the camera focuses in on Ranger Red's morpher with a blinking red light.
The Sanzu River from Power Rangers Samurai is one to the Nighloks, as nobody can last long away from the river before they dry out. Only a monster and some Mooks are able to go out at a time, trying to increase peoples' despair and sorrow in order to make the Sanzu's water level rise until it seeps into and floods the human world.
Master Xandred was sealed in the river by Jayden's father. Due to the seal being done wrong (which stems from the previous Red Ranger having insufficient power at the time), it causes him headaches.
"The Master Returns" seems to imply that he's still trapped to some degree, as flashbacks show he could previously leave the Sanzu River without drying out, a problem he has now.
Clark manages to escape on his own, without help from the outside, and subsequently reimprisons Zod in the season six premiere. He then spends the rest of season six hunting down the other Phantom Zone prisoners who escaped at the same time he did one by one and either killing or preferably reimprisoning them.
Despite the frequent use of the "Pandora's box" analogy, it was really Earth that was sealed in a can at the beginning of Stargate SG-1. On the other hand, there are several examples of villains being effectively sealed in cans, such as Osiris (who was in a canopic jar), Anubis' son (in a stasis chamber), the Wraith (hibernating), and the Ori (in a distant galaxy blissfully unaware that free humans existed).
One of the most literal Stargate SG-1 examples would be Hathor, who had been sealed in her sarcophagus for 4,000 years, only for some unwitting archaeologist to open it.
As well as Marduk. He was so bad by even Goa'uld standards that his servants locked him in a sarcophagus with a ravenous predatory beast. As it continually ate him, the sarcophagus' nigh-magical healing powers would keep healing him and see to it that it took him centuries to die. Damn. However, the Goa'uld are Puppeteer Parasite, so Marduk simply pulled a body-switch and took over the predator. And then Our Heroes go poking around in there...
The Replicators were sealed in a can by the Asgard, but it gets worse when the Replicators outwit the trap and crank it Up to Eleven before SG-1 arrive to fix the can.
Data's evil "brother" Lore in Star Trek: The Next Generation. He's found disassembled in Dr. Soong's lab and the Enterprise crew make the mistake of putting him back together.
Well, in the animated version episode "Beyond the Farthest Star", Kirk tricked an ancient evil entity onto a black dwarf star, where it moans about how lonely it is as the credits roll.
This was supposedly the plot behind the Star Trek: Voyager episode "Dragon's Teeth", when Seven of Nine releases an alien race from a 900-year stasis... only for them to turn out to be your bog-standard Villains of the Week piloting obsolete spaceships. Disappointing.
Another episode played off the Real Life example (see below) of nuclear waste- Data is injured while retrieving said material from a crashed Starfleet ship, and after the ensuing memory loss he stumbles into a local village. His radioactive-shielded briefcase doesn't stay closed for long.
Supernatural has used so many variants of this it's the only kind of Myth Arc they've ever had, but at least it always varies. Thus far the end of every even-numbered season has involved letting some great evil out of its can. Seasons three, five, and seven have been about dealing with these evils.
Azazel's goal in the season 2 finale was to open a Devil's Gate in Wyoming in order to release Lilith and a demon army into the human world. Lilith would be needed to later break Lucifer out of his cage.
In season 6, we have Purgatory, the place that creatures who don't belong in either Heaven or Hell go when they die. This seems to make it a repository of monster souls; it also contains a being known as the "Mother of All" which they call Eve, probably because they already used the names Lilith and Tiamat for other purposes, and apparently some very nasty formless things that were never alive in the normal sense. Cas and Crowley work together to open it in order to use the monster souls as a power source, but there are side effects. The first episode of the seventh season reveals that Purgatory was originally created to seal away the Leviathans, beasts that God feared would (as Death put it) "chomp the entire dish".
The first Torchwood season finale features Abaddon, son of The Beast who was sealed within the Cardiff Rift.
In The Twilight Zone TOS episode "The Howling Man," the evil sealed in a cell was the Devil himself.
In Tracker, Zin gets sealed in an underground vault during his attempt to steal the alien weapon hidden there.
Katherine and two dozen other vampires in The Vampire Diaries were sealed in a tomb beneath the town church. In a slight subversion of the usual formula, this was actually an attempt to SAVE Katherine from the town's vampire hunters.
Actually it was Katherine's plot to kill the tomb vampires because they knew she was alive and eventually word will get back to Klaus and he would find and kill her for his sacrifice. So she plotted with George Lockwood in 1864 to have them take the blame for the killings George was doing. George was a werewolf and he was killing all those people. So their interest met: The tomb vampires would take the blame for George's killings and at the same time be eliminated to keep Katherine's secret from Klaus. Katherine's "payment" for this was the moonstone George had.
The dagger dipped in white oak ash only kills the Original vampires if it stays in place. Klaus keeps all his siblings in coffins with daggers in them. He wakes up Rebekah when he needs her necklace.
Mikael. Katherine wakes him up, but no-one except Damon and Stefan is aware of this yet.
Esther, the Originals' mother who initially turned them all into vampires. Klaus tore her heart out and has been keeping her in a coffin, thanks to a spell that preserved her body perfectly.
It's debatable whether Peter Kramer was evil per say or just driven crazy by the Drake, but in any case, his spirit was locked inside a suitcase, which was then locked in a storage room in the basement that was bricked shut.
The Stone Tape. The scientists investigating the Living Memory of a Screaming Woman imprinted into the stonework of a Haunted Castle accidentally 'erase' the scream. They give up their research, thinking they've blown their chance, and refuse to listen to the sole member of the team who insists the scream was 'recorded' on top of a more ancient evil, which is now free.
The Dragon Lord and the Rank from Ninja Turtles: The Next Mutation, who were imprisoned in an enchanted mirror until they took the opportunity to break free, menacing the Turtles and replacing the Shredder as the Big Bad.