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 and used as part of his Xanatos Gambit to invade Earth and frame Sleepwalker as the invasion's leader.
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 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?
The Transmogrofier 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.