...this button-down, Oxford-cloth psycho might just snap, and then stalk from office to office with an Armalite AR-10 carbine gas-powered semi-automatic weapon, pumping round after round into colleagues and co-workers. This might be someone you've known for years. Someone very, very close to you.
The Puella Magi Madoka Magica doujin "I'm working at a mahou shoujo recruitment company, but I think I may be at my limit" has Kyubey going on one of these near the end, putting his workplace to the torch and killing everyone. Being screwed out of good work by his only friend in the workplace (when he's already in danger of being fired) and then finding out that he's been doing the nasty with his wife on the side and that the kid she's carrying isn't his was just too much for our favorite Starfish Alien.
Demon Knight. Wally. He was going to blow up the post office for the hooker he loved.
The main characters in Office Space joke about doing this early in the film, in order to establish just how terrible their jobs are. One of the minor characters actually does.
Fight Club: The Narrator threatens his boss that this might be the consequence if he pushes too much when his boss discovers the photocopied "Rules of Fight Club".
Jumanji: when Van Pelt goes to the gun shop, the salesman's reply is "You're not a postal worker, are you?"
Parodied in the third The Naked Gun movie. While Frank is dealing with a number of threats (itself a parody of The Untouchables), a woman screams "disgruntled postal workers!" and he sees a number of mailmen firing assault rifles.note "Disgruntled" was the media's preferred euphemism for going postal at the time.
Deconstructed in Falling Down. Granted, he doesn't exactly do it on the job, as he was fired a month before the plot kicks off, but the film is a story about "D-Fens", a recently divorced and fired white-collar worker going through a nervous breakdown, letting out his feelings of anger and alienation against a cruel, feckless world, and the Jerkasses who inhabit it. He accumulates a number of weapons through chance and goes on a rampage of destruction through Los Angeles. His character is contrasted against the Hero Antagonist Prendergast, who faces the same problems on the job but deals with them with maturity and empathy. Eventually, D-Fens devolves into a Sociopathic Hero, has a massive Heel Realisation after Prendergast easily cuts down his whining and justifications, and eventually commits Suicide by Cop when he realises the damage he's done.
One episode of Criminal Minds dealt with a workplace killer willing to do anything to avenge himself on the pharmaceutical company that (from his perspective) had abused him for years.
The phrase is humorously discussed by Rossi in one episode. He tells Prentiss about a former brother-in-law of his who was a mailman and how he would deliver a mountain of mail all day, no matter what conditions, and the next day, he had to do the same thing all over again. According to Rossi, the phrase "going postal" made perfect sense to him.
One of the group reaps in Dead Like Me involves this scenario. We don't see the actual shooting, and it (somewhat surprisingly, for this show) isn't played for laughs.
Another episode mentions how Newman recently picked up Ted Bundy's old postal route - he'd never seen so many dogs on one route before.
Parodied in the MADtv sketch "Postal Workers Gone Postal". Two postal workers decide to have their murderous rampage on the same day, but of course they don't want to share. They argue about who gets to go on a killing spree, which has the more traumatic past, and which person each of them gets to kill. Then a third guy who was planning the same thing walks in. When a fourth guy walks in, they ask him why he hates being a postman, but he turns out to be a robber. They use their guns to arrest him and are hailed as heroes, and the government makes firearms mandatory for all postal workers.
Feng Shui has the 'Consumer On The Brink' character type, for players who want to play one of these in an action-movie setting.
Dead Rising featured Carl Schliff, a proud member of the United States Post Service. Before the zombie apocalypse, he had a perfect record at his job. Once he learns who Chuck is, he goes into a homicidal rage, and starts throwing around bombs at him.
"Neither rain nor sleet nor hail, NOR ZOMBIES are going to keep me from my appointed rounds!" - Carl Schliff
Boyd Cooper from Psychonauts both parodies and plays the trope straight, being a man who burns down the department store of his former security-guard job with molotov cocktails made out of milk bottles. His mental condition is only cured when he burns down the mental institution for which he worked as a guard.
In Afterlife, one of the Ironic Hell punishments for Wrath is "The Post Office Game", which makes the damned do postal service in Hell, forcing them to put up with wrong addresses, weird smells, and the occasional explosion. This punishment also puts some damned with packages on endless lines, where the attendants close for lunch every time they can attend someone. Conveniently-placed rifles are found on both sides of the punishment.
Destroyman from No More Heroes was a post office work for his civilian disguise, fittingly enough he's also by far the single most deranged assassin in the game (and possibly the series).
Rocko's Modern Life has a postman revealing he was laid off and feeling disgruntled. The other passengers quickly vacate the car... only for him to swing around as he wanted space.
An episode of "Justice Friends" briefly features a Joker-like villain named Disgruntled Postman, who robs a post office to steal a single postage stamp to mail a bomb to the US President.
Referenced in a 1999 episode of The Simpsons while on a field trip to a post office, Nelson asks a worker if they go on a shooting rampage. He replies those days are over. Skinner chimes he's glad he works in a school (which loses taste as a trend of students going ballistic in schools and aired just months before the Columbine Incident).