Benevolent Precursors

The exact opposite of Abusive Precursors: The Sealed Evil in A Can is clearly locked and labeled in every language they could think of, complete with handy pictures showing stupid mortals getting killed by said Sealed Evil, just to make sure the message gets through. And even if you DO manage to get through the five thousand doors locked with The Power of Love and release the demon god, don't worry--they had a fail-safe in place where the seals on the one weapon capable of destroying it would be unlocked at the same time. Sure, these guys died out millennia ago, but at least they did their best to keep us from following them to the grave.

Obviously, these Benevolent Precursors probably fail in the end; if your Sealed Evil in A Can stays sealed, you don't have much of a plot. If they come back with their super-tech to help the heroes later, it is Awakening the Sleeping Giant.

Compare Neglectful Precursors and Abusive Precursors.

Comics

 * All of Earth's Pre Crisis superheroes to the current DC Universe especially the Monitor. They fought and defeated the Anti Monitor and sealed a single universe inside the essence of the Monitor. Then the universe was restarted twice with big bangs. None of the Post-Crisis stories would have happened without the Pre Crisis heroes. Infinite Crisis then turns this into a strange case of the Benevolent Precursors becoming the Sealed Evil in A Can.

Film

 * The Mondoshawans ("Monduchivans" in the language originally spoken by Leeloo) from The Fifth Element. They set up a secret order on Earth to safeguard the elements needed to fight the Ultimate Evil. They came back to remove the elements to a safer place when World War One threatened them, and they were in the process of returning them when they were shot down by Mangalore warriors flying illegal starfighters supplied by Mr. Zorg.
 * Zi Yuan from The Mummy: Tomb of the Dragon Emperor. She is a sorceress who seals the Big Bad sorcerer-emperor Han and his Badass Army, then gains Immortality, creates a weapon to kill the Big Bad if he is ever unsealed, makes friends with Yeti to protect the one place Immortality can be gained, and is still around to summon her own army conveniently sealed near Han's own. If this doesn't sound impressive, then compare it to the ancient Egyptians in the first two movies whose sealing of Imhotep gives him powers and Immortality despite being perfectly capable of killing him normally.

Literature

 * In L.E. Modesitt's Forever Hero series, the last survivors of civilization sealed away their nuclear and biological weapons as the Earth was dying, with a warning to not try to open the vault.
 * Lensman has the Arisians, who effectively countered the Eddorian meddling with pre-spaceflight Earth, provided the predecessor of the Galactic Patrol with a better FTL drive, created the Lens, which provided the Patrol with the means to effectively fight the Boskone drug-dealers, instituted a secret breeding program which eventually culminated with the Children of the Lens. And they did all this in the full knowledge that the Children would surpass them in mental power.
 * The Inheritance Cycle has the Grey Folk, a vanished race of magicians that lived in Alagaesia before the Elves arrived. They were powerful magicians but couldn't fully control sorcery because it was entirely nonspoken---a stray thought could cause a disaster. After a magical cataclysm that they had caused devastated Alagaesia, they sacrificed nearly all of their power to create the Ancient Language. According to Paolini, there are no pure Grey Folk left, but it would seem likely that  are Half Human Hybrids descended from them due to their longevity and propensity for unspoken magic.
 * In the novelization of the film Alien, explicitly states that the Space Jockey aliens (in whose ship the alien eggs are first found) set up the transmission the Nostromo received as a warning to other ships to stay away (a fact that  discovered only too late to avoid disaster), and calls them "a noble people" for leaving said warning. Given what we have seen of the trailers for Prometheus, it is possible (even probable) that this 'canon' statement could end up being Jossed.

Live Action TV

 * In the new Doctor Who two-parter "The Impossible Planet"/"The Satan Pit", the Monster of the Week, who's implied heavily to be Satan is
 * They also get credit for successfully
 * Farscape: As seen in the Peacekeeper Wars post-series finale, the Sealed Good in a Can Eidolons. They can lull any other species into a receptive state, and read their minds, to construct a good arguments, and help factions co-exist. Besides being basically Super-diplomats, they are the ones that  They came back to the galaxy thanks to a reversal of their Critical Existence Failure.
 * Star Trek
 * The unnamed ancient humanoids from "The Chase" who seeded planets with their DNA, thus being responsible for all humanoid life. They also encoded a map in this DNA that led not to a superweapon, or a species of Omnicidal Maniacs, but to a recorded message expressing the hope that their "descendants" would cooperate with each other to discover the message. Leads to a rather epic guilt trip for Picard and (apparently) the Romulan commander.
 * The Preservers, who transported endangered cultures to pristine worlds.
 * In Star Trek Online, the Preservers are  with their descendants.

Video Games

 * The Jjaro of Marathon. After the Jjaro's disappearance,
 * The Forerunners from Halo. Yes, they used galaxy-wide superweapons to destroy all life in the galaxy, but they really did try everything else first (getting to the point where obliterating Flood-infested worlds from orbit was a first resort, and their tactics got progressively more destructive from there) they just failed. They also clearly labeled the Flood and the Halos as dangerous--the Covenant just ignored them. After they fired the Halo Array, they even repopulated the galaxy with all the lifeforms they had indexed so that the galaxy wouldn't be devoid of sentient life.
 * In truth, their benevolence has also been blamed as a form of negligence. By preventing wars and encouraging disarmament, the Forerunners left both themselves and their protected races unprepared for the Flood.
 * If the only evidence you have is the first and part of the second Halo games, they do appear either neglectful or outright abusive. Once you learn more, it brings their actions into a fresh light.
 * The Forerunner Trilogy makes it clear that they were  To be fair, the second book implies that.
 * In Mass Effect the Protheans
 * Slightly subverted in Mass Effect 3. reveals that the Protheans were actually a Proud Warrior Race that dominated the galaxy before the Reapers arrived, and they were mainly trying to preserve their empire.
 * This subversion is debatable, given that this understanding is based on a Proud Warrior Race Guy born 50 years into the war with aforementioned Reapers and admitted to being a product of his time. Depending on what Alternative Character Interpretation you buy into about the Protheans, given the new information on them
 * The Sosiqui created the Legacy of Time in Journeyman Project 3 did their best to educate their chosen successors, humanity, in how to care for their gift. Even with the interference of other aliens, humanity managed to keep their legacy to the universe intact until a time would come when all races could utilize it in harmony.
 * The Precursors in Star Control II, with some things. That bomb of theirs is labeled clearly enough for both the Utwig and the human scientists to figure out what it is. Their starship factory is user-friendly, and a certain other artifact is well-shielded.
 * The Taalo leave behind a device that prevents evil alien mind control. This is instrumental in saving the galaxy.
 * Most of the upgrades in Metroid Samus Aran finds during her adventures have been left there by the Chozo, who foresaw that she would be needing them in the future. With Samus's relationship with the Chozo taken into account, this could almost count as an inversion of Abusive Parents.
 * The Masari count towards this in Universe At War, guiding humanity's development and watching over them. When Earth is attacked by the game's Big Bad several alarms are tripped and the Masari begin coming out of hibernation The results were predictable.
 * In Final Fantasy VII, when some cosmic force gave the Cetra the Black Materia, a magical stone capable of destroying the entire Planet, they transmuted it into a form in which it could not be used: a giant pyramid-shaped temple. (Just go with it.) The temple cannot be entered without a Keystone, which was supposedly hidden after the temple was finished. The temple itself is populated with bloodthirsty monsters and avatars of the Cetra to deal with trespassers. The room containing the device for un-transforming the temple has a sequential mural depicting the shit that's gonna go down if it gets used. As a last resort, if the Temple is changed back into the Black Materia, it crushes anybody inside, including the person that activates the device.
 * The Orions in Master of Orion. They sealed away the Antarans in a pocket dimension and left their own homeworld protected by the Guardian, in order to keep away races that weren't ready to handle their super-powerful technology.
 * Those Who Came Before in the Assassin's Creed series started out pretty abusive what with creating and enslaving humanity with their Pieces of Eden. The combination of a slave revolt and a solar flare that nearly destroyed all life on Earth changed that. Before dying off, Those Who Came Before worked together with humanity to rebuild the world, and they left behind tools and knowledge (in the form of the Pieces of Eden) to give humanity a fighting chance when the next solar flare hits Earth.
 * Played with in the X-Universe. The Ancients theoretically do have good goals, such as preventing the heat death of the universe, and they consider the Portal Network they built a gift to the younger races. Unfortunately they have a tendency to think of the younger races as a single group (not true), making them frighteningly willing to toy with other species (see Abusive Precursors).

Real Life

 * Truth in Television: Us, hopefully, with regard to our own Sealed Evil in A Can, nuclear waste. There are numerous people planning how to store the waste (which will be harmful for hundreds of thousands of years) in a way that will protect and warn our successors of its dangers no matter what.
 * Further explanation: A site in Nevada called Yucca Mountain was studied as a possible tomb for the US supply of radioactive waste. The government researched how exactly one could build a clear "Do Not Open" monument that would last for thousands of years and still be understood. For numerous reasons including Nevada's objection, disputes over nuclear policy, and questions about geological stability, the project seems likely to be abandoned. At least it shows we care.
 * Some scientists have joked about hiding the nuclear waste behind multiple layers with multiple signs to remind the person trespassing to not go any further. They joke that if one of our descendants are dumb enough to go past that many signs then it will just be natural selection doing its job to rid the world of that idiot. Hopefully that won't happen as funny as it may sound now. Nuclear waste lasts for millions of years. We might well have died of plague, global warming, war or anything else. Whatever comes AFTER us wouldn't necessarily understand any language spoken today. Even if you include the idea that perhaps a circle with a slash through it universal the creature might be sufficiently non-human to view those warnings as not applying to whatever it is.
 * Ironically, one of the more popular solutions is to simply bury it deep underground in the middle of nowhere, shielded extensively by sturdy, radiation blocking material. By the time some future civilization finds it and is actually capable of reaching it through the sturdy shielding, they're probably capable of understanding that it's dangerous stuff with no practical purpose beyond "killing everyone who comes into contact with it"--likely via some kind of radiation detection. This whole setup has merit because any future civilization might completely misinterpret the warnings--designed to be clearly visible and obviously artificial--as something glorious, powerful, Lost Technology, or something else, due to a combination of ignorance, stupidity, cultural differences, desperation, etc. Or they just might ignore the warnings altogether. Thus, obscurity may just be better than any explicit and overt warning. Granted, if it were something like The Virus or Sealed Evil in A Can (in the living being sense, not a "useless radioactive waste" sense), we'd likely outright obliterate the damn thing or hurl it into the sun. And provide Ragnarok proofed instructions on how to counteract it should it somehow appear anyway.