The early 1990s Marvel Comics series Sleepwalker featured the title alien's Imaginator, a teleportation device that can be used by the Sleepwalkers to teleport almost anywhere they can imagine, and to imprison the monsters they capture. Sleepwalker becomes trapped in Rick Sheridan's mind when Rick mistakes the Imaginator for a weapon and takes it away from him, before the device is later retrieved by Cobweb for the invasion of Earth and framing Sleepwalker as the invasion's leader.
In the second issue of Secret Six Villains United, one of the miniseries leading up to Infinite Crisis, we see a pair of weapons mounted on Scandal's wall when she's writing a love letter. It's the first subtle hint that she is a Dark Action Girl instead of the non-combatant Middle-Management Mook she had appeared to be to that point.
In Supergirl (Vol. 5) issue #38, someone sends Cat Grant a Supergirl doll. In issue #58 Cat realizes that it's a clue to find a child kidnapper.
In the sixth issue of Supergirl (Rebirth) the suit of Strange Visitor -a female hero who had not make a comic-book appearance in more than one decade, real time- can be seen in the background. In issue #15, Strange Visitor makes her first proper appearance.
Early in Courtney Crumrin and the Night Things, Courtney reads her poetry in front of her class. Unbeknownst to her, her poetry reading conjures up a great storm and creates clouds so black they block out the sun. And no one mentions it after she's done for the rest of the series. Until the final issue where a memory-wiped Courtney is given her poetry book, which returns her lost memories and magic, and gives her access to a spell strong enough to overcome an entire government body of wizards.
A cloneworks for xeno-anatomy and a villain with innate power-nullifying abilities both showed up early in the latest volume of Empowered. Both of those and the suit becomes invisible, wearer does not trick from an earlier collection become major factors in the last chapter.
A coffee mug labelled "World's Best Dad" appears on a cluttered desk in one issue of The Invisibles. Several issues later, it's used to save the day.
The information pollen in Transmetropolitan, which seems to be just one among the many random, wacky elements in the story but which gives Spider a degenerative brain disease.
The appearance of Mister Mind in the first issue of 52. He's mentioned off-hand maybe twice after that, and then disappears for almost fifty issues before reappearing in the penultimate chapter, having been revealed as the Big Bad.
In the very first issue of Big Bang Comics, Kid Galahad reads a book about escape artists and magicians. Later, when he's held captive by the Quizmaster... you can probably guess what he does (eventually - he was playing it close to his vest at first).
In Final Crisis Aftermath: Dance, Most Excellent Superbat mentions that he kept a souvenir from the "Brain Drain" escapade - the control cell that turned the team microscopic. You never know when you'll have to be really tiny... so naturally it comes up in the last issue.
In the Tintin book Tintin: The Castafiore Emerald, Tintin and Captain Haddock spotted a magpie in the front yard of Marlinspike very early on in the book. Later in the story, Bianca Castafiore's titular emerald was stolen. It turns out the thief was the magpie.
In Under the Hood, the Red Hood has Chekhov's RPG and Chekhov's spare mask in his HQ room.
Chekhov's seabird: in The Boys story arc Highland Laddie, a visitor tells Wee Hughie about the fulmar's real-life defensive projectile regurgitation ability. Later, a local crime boss that Hughie is pursuing disturbs a fulmar nest, and gets a shot right in the mouth.
Early in Bookhunter, Special Agent Bay observes a library during operation hours. Many of the objects and locations he examines in this scene—the card catalogue, the moving bookshelves, the front display window, the anti-theft alarm system—end up being used as weapons when a fight occurs in this same library.
In an issue of IDW's G.I. Joe, several Joes are exploring a disused U.S. military storage facility, where one of them spots an Awesome, but Impractical Cold War relic - an M65 "Davy Crockett" Nuclear Rifle (i.e., an atomic bazooka). Guess what weapon gets taken down from the wall (well, uncrated) during an unexpected attack by Cobra?
In Sonic the Hedgehog, Dr. Eggman and Snivley steal the Blue Chaos Emerald and use it for Operation Clean Sweep. However, Sonic uses Chaos Control and fixes the damage, causing the Chaos Emerald to disappear. Two years later, it reappears... in the possession of Dr. Wily.
Parodied in My Little Pony: Friendship is Magic (IDW). Pinkie claims that since she lugged those "bulky" costumes all the way from Ponyville, they have to be useful at some point in the quest. While Rainbow's costume never serves a meaningful purpose, Pinkie's costume is animated to keep an eye on the Changelings after their defeat.
Could double as Chekhov's Gag, considering the fact that both times before the climax it's brought up, it's for humor purposes.
Played straight in the second story-arc, Spike bringing the Fire Ruby and his unexplained immunity against the Nightmare Forces are two giant, flashing guns hanging on the wall.
In #7, the Fire Ruby helps Spike expose Nightmare Rarity'sLotus-Eater Machine as a lie, since dream!Rarity doesn't recognize it. Also, despite dismissing his attempts to reach her as pointless, Nightmare Rarity keeps the ruby.
Rorshach's journal from Watchmen is an interesting example of this trope. While it is certainly an example of a chekhov's gun, its eventual fate is left uncertain. Its return is the last thing the reader sees when reading the comic, and we get no clue as to whether or not the trope will be played straight, averted, or subverted.
One of the many failed ploys to permanently kill Dracula in Dracula Lives involved a set of words called the Montesi Formula, a chant that could kill any vampire for good. The pages containing the formula eluded him in a Sequel Hook, and the formula returned in a story arc of Doctor Strange: Master of Mystic Arts, where they were used to kill all vampires in existence.
Transformers: More than Meets the Eye: Has numerous. Rung's thumb microphone was mentioned just so it could be later used in a hostage crisis. Magnus stressing the importance of a semi-colon comes into play when Tailgate rewrites the law to defeat the Legislators. Then there's Trailcutter, who get's guns installed in his legs and is revealed to deploy a force field as a panic bubble to save himself from harm. It saves him during an encounter with a DJD member only for another to get around his defenses and brutally kill him.
Serenity: Leaves on the Wind: One of the scientists who transformed River into a Tyke Bomb says she wasn't "complete", and sics Iris on them. Meanwhile, outside, the Operative surrenders rather than face Kalista. Kalista is another product of the same program that created River and Iris. Her Sci-Fi Bob Haircut is a wig concealing the same heavily scarred head as Iris has.
Lesbian Zombies from Outer Space: In Issue #1 Ace is searching for a video camera in a cabinet, and a shotgun conspicuously falls off one of the shelves. He pays it no attention, but the weapon resurfaces in Issue #2 when Gwen tumbles down the stairs and crashes into the cabinet, knocking the shotgun loose once more. And at just the right time.
Don Rosa's The Black Knight: In "The Black Knight Glorps Again", early on in the story Gyro Gearloose carefully makes a small opening in the block of concrete that the dissolving black knight armor was kept in because it continued to destroy the air molecules around it, creating a near vacuum that they would otherwise have been sucked inside of. Later on Donald and Lusene (who's wearing the armor again) are both locked inside a containment room with Lusene threatening to dissolve Donald unless Scrooge lets him out. Donald remembers what Gyro had told him earlier and keeps the black knight distracted long enough for the oxygen in the room to run out and knock them both unconscious so Scrooge and his nephews can safely remove Lusene's armor.
Pouvoirpoint: Aboard spaceship Entreprise-2061, the main character receives training to handle a rifle called Chekhov Gun, which will of course be dramatically useful in the climax scene...
In the horror series Clean Room a demon collects all the sharp objects in Chloe's house and positions them point-up in her bathtub. It looks like a lethal trap set for her, but it's really the demon offering her weapons in a place she'll see and remember when she's attacked minutes later.
Ultimate Galactus Trilogy: Dr. Phil Lawson goes through several ID tests before joining a top secret meeting. It was later revealed that he is an alien, who has gone through an extensive treatment to look like a human. And what a better way to make the alien technological advantage more clear than showing how he fooled the most advanced security network of the world?
The Transmogrifier Gun in Calvin and Hobbes. It was first used for a story arc where Calvin introduces it and wants Hobbes to turn him into a Pterodactyl. He turns him into a tiny one, and a massive transmogrifying fight ensues. In a later arc, Calvin is falling down to earth because a balloon that lifted him in the sky popped. He roots for some chewing gum in his pocket, in the hopes he can blow a big bubble and use it as a parachute, when he finds the gun, transforms himself into a light particle, and zips back home.