From the Batman universe, Harvey Bullock is usually one of these. As was late 1980s supporting character Joe Potato.
The nameless protagonist of Potter's Field by Mark Waid is another.
Hellboy is an otherworldly version of the noir classic model, a heavy-drinking, chain-smoking, cynical demon with Badass Longcoat who sticks his nose where it doesn't belong, takes a beating, etc. etc. He's often referred to as "The World's Greatest Paranormal Investigator".
The DC comic character Ms. Tree, created by Max Allan Collins, is a relatively rare female hardboiled detective.
Steve Ditko loved Hardboiled Detectives, and his two (very similar) characters Mr. A and The Question are objectivist takes on the Trope.
The title character of the Spanish comic Blacksad is a hardboiled detective in the 1950s — and a cat.
The titular character from 2000 AD's short lived gamebook/comic hybrid Diceman, Rick Fortune was a variation on this trope, being a hard boiled psychic investigator with a pair of magical dice that could summon a demon amongst other powers.
Dwight McCarthy of the Sin City series is a quite violent one of these, though he becomes more of a vigilante as the series goes on.
Hoyle from the surreal and cerebral Noir/SF crossover Yesterday Was A Lie is a distaff version, with fedora, trenchcoat and all, trying to find a missing scientist.
Louis Simo from Hollywoodland is a deconstruction loosely based on a real detective, Milo Speriglio.
The 1971 film Gumshoe, starring Albert Finney, features a London man who decides to adopt a Sam Spade-like persona to escape his boring life, and quickly becomes embroiled in a plot involving drugs, gun smuggling, and gangsters.
Raymond Chandler's Philip Marlowe, protagonist of The Big Sleep, Farewell, My Lovely, and other novels, is an iconic and much-copied example. Even the introduction to Marlowe in recent prints sums this trope up pretty well:
I'm a licensed private investigator and have been a while. I'm a lone wolf, unmarried, getting middle aged, and not rich. I've been in jail more than once and don't do divorce business. I like liquor and women and chess and a few other things. The cops don't like me too well, but I know a couple I get along with. I'm a native son, born in Santa Rosa, both parents dead, no brothers and sisters, and when I get knocked off in a dark alley sometime, if it happens, at it could to anyone in my business, nobody will feel the bottom has dropped out his or her life.
Carroll John Daly's "Three Gun" Terry Mack is possibly the Ur Example of this trope, predating Hammett's Continental Op by several months. Daly's Race Williams is also an example.
Archie Goodwin, in the Nero Wolfe series, was a partial deconstruction. Created during the trope's peak years, Goodwin had many of the classic elements, but he worked for Wolfe, the fat, home-bound Great Detective. Archie did all the footwork and fighting, but tended to avoid the cynicism and world-weariness of the true hardboiled detective.
Stout had another, much smaller and less popular series starring Tecumseh Fox, who was much more the straight hard-boiled type.
Mickey Spillane's Mike Hammer was an early, over-the-top, ultraviolent, Knight Templar example who is often credited with helping turn the genre into a parody of itself.
Invoked by Vincent Rubio in Anonymous Rex. He's a detective — and a velociraptor! He claims he's not really hard-boiled, but he acts like he is because that's what the customers expect. He even uses the "Bogart" persona to pick up female dinos.
Sara Paretsky's VI Warshawski is a distaff version of the (usually) male hardboiled detective.
Lazlo Woodbine, from the Far Fetched Fiction of Robert Rankin, is a blatant parody. He insists on using the first person, getting knocked unconscious at his first appearance and can only appear in four scenes (his office, a bar, an alleyway and a rooftop). Considering the outlandish nature of his books, often involving things such as time-traveling Elvis doing battle with Eldritch Abominations out to unmake existence, this makes things awkward.
His late mentor Sokichi "Boss" Narumi, on the other hand, had much moresuccess modeling himself on the Chandler-esque ideal of manliness. Chandler is name-dropped in The Movie, and Sokichi named the young man who would become Shotaro's partner after Philip Marlowe.
Ryu Terui, although a police detective rather than a private eye, comes rather closer to the trope than Shotaro, with comparison explicitly drawn to his being the kind of person Shotaro aspires to be. However, all that goes out the window in his Roaring Rampage of Revenge quest to find the man who murdered his family. Thankfully, interacting with Shotaro and Philip helps Ryu level out, eventually leading him to marry Sokichi's daughter Akiko.
In the noir-esque South African Sci-Fi thriller, Charlie Jade, Charlie is an homage to the older Chandler/Hammett style of hardboiled detective. He even sports the classic trenchcoat (though no fedora), and uses the Private Eye Monologue.
Peter Gunn made from 1958-60, was a Hardboiled Detective with a 50s Jazz cool to him.
The main cast from the supernatural neo-noir series Angel act as a general deconstruction of the trope, although play some parts to a T.
In Calvin and Hobbes, Calvin's imaginary alter-ego, Tracer Bullet, is a pure parody of the hardboiled detective.
"I have two magnums in my desk. One is a gun, and I keep it loaded. One is a bottle, and it keeps me loaded. My name is Tracer Bullet. I'm a professional snoop."
Garfield occasionally features Garfield as Sam Spade. Due to his being a cat, however, having people ask "Spade*
?" tends to get a "Why do people keep asking me that?" in response.
In the Cabin Pressure episode Uskerty, Arthur and Douglas are drinking in an airport bar, and Arthur tries to channel this trope.
Arthur: Hey you guy. The dames,eh? Yeah the dames. Stupid dames. Do you have any luck with the horses? No, the horses are all idiots. You know between the dames and the horses sometimes I don't even know why I put my hat on. That's how we talk in bars,isn't it?
Douglas: No, Arthur. That's not how anyone talks...anywhere.
A Call of Cthulhu scenario included "Artie Gumshoe - Tough Private Investigator" as a pregenerated character, packing a .45 Automatic and with an illustration showing him with a cigarette wearing a fedora and trench coat, inviting him to be played like this trope.
Parodied in Problem Sleuth, where the main characters think they are this, and occasionally do things like practice their hardboiled monologues or are drawn in Chiaroscuro. From the reader's perspective, they act more like unspeakably, unspeakably silly Eastern RPG characters.
Lewton in Discworld Noir both embodies and parodies this trope, due to the Disc's Theory of Narrative Causality; he doesn't know why being a private investigator means he has to wear a trenchcoat and fedora, but he's quite sure it does.
Booker DeWitt, the protagonist of BioShock Infinite, is a hardboiled ex-Pinkerton PI sent on One Last Job to clear the debts of his gambling addiction. Unlike most hardboiled detectives, however, his milieu is the bright, shiny, blue-skied floating city of Columbia (although you barely have to scratch the surface before realising how screwed up that place is, and that's before finding out about its real secrets...).
Max Payne borrows just as much from Hardboiled Detective fiction and Film Noir as it does the Heroic Bloodshed genre. In the first two games, he's an actual police officer as opposed to a private detective, going on a Roaring Rampage of Revenge. The third game plays the spirit of the trope a bit straighter, however. Max is no longer a police officer and works as private security, and spends most of the game trying to rescue a Damsel in Distress and overcoming his alcoholism. Though, the setting switches (mostly) from gritty and dark New York to bright and vibrant São Paulo.
The Fairly OddParents in Where's Wanda; Timmy wishes to become such a detective after the disappearance of Wanda, and ends up spoofing Sam Spade and Rick Blaine.
In The Venture Bros., Hank gets a fedora and affects a classic hardboiled detective personality whenever he's wearing it. It gets him laid for the first time.