Sam Spade doesn't mind a reasonable amount of trouble.
Down these mean streets a man must go who is not himself mean, who is neither tarnished nor afraid. The detective must be a complete man and a common man and yet an unusual man. He must be, to use a rather weathered phrase, a man of honor. He talks as the man of his age talks, that is, with rude wit, a lively sense of the grotesque, a disgust for sham, and a contempt for pettiness.
A seeker character frequently used in Detective Drama. A professional detective not directly affiliated with a police department in any official sense (although many will have contacts in the department, and it's not uncommon for members of this profession to have been either police officers or previously worked in law-enforcement, as many of the skill sets overlap), a Private Detective takes on cases that private citizens bring to them - however, whilst they aren't supposed to investigate crimes (which are official matters for the police, who often look dimly upon private detectives sticking their noses in - both in fiction and in real life), they usually find themselves knee-deep in murders, robberies and kidnappings by the end of the story. They may be doing this because the Police Are Useless and/or unconcerned about solving the case, meaning our detective is the only person who is actually willing or capable of solving it. However, this is often justified (especially in classical Film Noir) by the detective starting off with a seemingly simple case, such as finding out if a woman's husband is committing adultery or investigating the disappearance of a man who vanished twelve years ago in order to resolve a debt he had with a wealthy businessman, only for things to spiral out of control to the point that the only way for the detective to get out is to solve the case.
Although it's more common for a Private Detective these days to be treated as just one step away from the Amateur Sleuth (or often, particularly if seen from the point of view of the police, as rather sleazy bottom-feeders usually involved in some kind of criminal activity and frequently ex-cops kicked off the force for some kind of corruption), the classic Archetype of the Private Detective - and the one that has generally stuck in the mind of people when they think of the profession - is either the Great Detective or the Hardboiled Detective.
While the comparison with the Amateur Sleuth is common, in many ways the stories work the opposite ways. The Amateur Sleuth, such as Miss Marple, tend to cozy locked room mysteries, where everything starts complicated and uncertain, but slowly works its way down as a logic puzzle, with a tidy solution where the killer is unmasked. The Private Investigator tends to start simple, but as the investigation will unravel an ever more complicated plot, and the ending is rarely tidy. The killer may be unmasked, but larger problems tend to stay unsolved.
Vampire Detective Series often feature one as a protagonist, though of the immortal variant.
Detective Conan Kogoro Mouri, Heiji, and main character Conan Edogawa/Shinichi Kudo
In fact, Kogoro has often been shown to do stuff that people usually hire private detectives for - tracking people suspected of Adultery, trying to track down someone who is lost (but not officially considered a missing person, think deadbeat dads or former friends.)
One of them in Japan Inc finds out that a big union boss is a customer in an S&M club.
Heironymous "Hip" Flask is a private eye and anthropomorphic hippo in the Elephantmen comics.
From E-Man comes Michael Mauser. Just... don't call him Mickey.
Judge Dredd spinoff The Simping Detective features Jack Point, a judge working undercover as a private detective. In clown gear. Simping is a fashion trend/sub-culture in Mega City One. The point of the trend is to look as stupid as possible so that people will bother to notice that you exist.
Dwight from Sin City. His career comes to an end once he's wanted for murder, however.
In the Sam & Max universe, the main characters are rather atypical private detectives (though they prefer the term "freelance police"), while their neighbor Flint Paper is a more stereotypical, two-fisted, Dirty Harry type.
Ms Tree: She and her husband owned a private detection firm; she's carried on since his death.
A 1975 film The Black Bird is a comedy sequel to The Maltese Falcon with George Segal playing Sam Spade, Jr.
Casey Affleck's character in Gone Baby Gone is an example of a modern PI.
Kiss Kiss Bang Bang has fun with all the Film Noir tropes, this one included: Val Kilmer's character 'Gay' Perry Van Shrike is a hard-bitten, tough-talking, gun-slinging Private Detective who's also, well... gay. And considers his job very boring. And, at least until Harry and Harmony come into his life, isn't exactly dogged in his pursuit of justice.
In Lethal Weapon 4, Leo Gets becomes a licensed PI. This garners much comedy for Riggs and Murtaugh.
Riggs: "Excuse me, private investigator? Could you investigate my privates?"
Shaft: Who's the black private dick, that's a sex machine to all the chicks?
In The Big Fix, Richard Dreyfuss plays the most true to life private detective ever filmed. Like real private detectives, he spends most of his time on divorce cases and (mostly legal) corporate espionage and commercial investigations. For the movie he does deal with a murder, but there is no doubt it's the first one he's come across in years of detective work.
In keeping with its inspiration from classical noir, Angel Heart centres around a classical private detective, though he doesn't wear the suit and fedora as much as some of the others on this list. Unfortunately, the things he ends up dealing with are probably a heck of a lot more horrific than the other people on this list.
In There's Something About Mary Ben Stiller plays, Ted a modern day private detective who is hired to stalk on the title character for a ex-lover in high school. Ted averts the honorable portion of the trope by going after Mary for himself.
Older Than Radio: Sherlock Holmes is often described as the 'first consulting detective.' Private detectives existed prior to Holmes, but he claimed that being a 'consulting detective' was something different:
'Well, I have a trade of my own. I suppose I am the only one in the world. I'm a consulting detective, if you can understand what that is. Here in London we have lots of government detectives and lots of private ones. When these fellows are at fault, they come to me, and I manage to put them on the right scent. They lay all the evidence before me, and I am generally able, by the help of my knowledge of the history of crime, to set them straight.'
Joe Sixsmith, a character of crime fiction author Reginald Hill, subverts most of the basic Private Detective characteristics: a short, balding, middle-aged, black private eye from Luton, whose hobbies include singing in a choir and motor mechanics. His main talents are being a nice, sympathetic sort of guy, knowing when he doesn't know things, and tremendous serendipity.
Robert B. Parker's Spenser is a modern, politically correct version of the type. If anything, he errs as far on the "sensitive" side of the "sensitive tough" archetype as Mike Hammer errs on the "tough" side.
The titular character of the Joe Copp series, written by Don Pendleton (also the author of The Executioner series), is a private detective who used to be a cop.
Mma Precious Ramotswe (in the The No. 1 Ladies' Detective Agency series by Alexander McCall Smith) subverts almost all of this trope, being a kind and overweight Botswanan lady who solves everyday problems, like absconding husbands, by a sharp psychological perception, persistence and being able to win the confidence of others.
Nohar Rajasthan from S. Andrew Swann's Moreau series of books is a down-on-his-luck PI... who just happens to be an 8'-tall humanoid tiger Super Soldier.
In L. Jagi Lamplighter's Prospero's Daughter trilogy, Mab's body was modeled after Bogart, and he acts as the company detective.
Rex Stout's Nero Wolfe was a somewhat sedentary stay-at-home version of this; narrator Archie Goodwin did most of the legwork, and if they needed more legs they'd hire three other private investigators.
In the children's mystery series, Trixie Belden, the title character and her bestfriend, Honey, plan on opening the Belden-Wheeler Detective Agency when they're older.
Helen Walsh in The Mystery Of Mercy Close is one as well as a Defective Detective in that she's severely depressed but still does her job anyway. It's mentioned by her sisters that Helen went through a variety of jobs before settling into her career as a private detective.
Angel started out as a private detective, before he made the transition from "supernatural crime" to "supernatural". The trope is subverted in that Angel and his True Companions are better at fighting demons, and sometimes have to hire a real PI when actual investigating is required.
Humorously parodied in the Canadian TV series Butch Patterson: Private Dick. Butch is given to internal monologues, wears a fedora everywhere he goes, refers to himself as a "Dick", and drinks very heavily...so heavily, in fact, that he's known to continually wet his pants and prematurely ejaculate. To make matters worse, he's also got a thing for prostitutes, a tendency to wake up in strange places after passing out drunk, and it's unlikely he'll ever live down that incident at the petting zoo. In spite of this, he's actually a very competent detective, and generally manages to solve the case, although he quickly blows whatever money he makes on pornography and whores.
A somewhat early TV example: John Cassavetes' piano player turned "jazz detective" Johnny Staccato, in the eponymous 1959 show .
Moonlight's Mick St. John started out as a private detective, before he made the transition from "supernatural crime" to "supernatural".
An episode of NCIS has the team working with a private investigator. Gibbs expresses his contempt for the fellow by repeatedly referring to him as a "private dick," emphasis on the second word.
Shotaro Hidari of Kamen Rider Double is a Private Detective. However he isn't as hardboiled as he likes to think he is, leading to his fellows referring to him as "half-boiled". Both major characters refer to themselves as "two detectives in one": Shotaro does the field investigations while his partner Philip (named after Philip Marlow) does the research back home. Prior to the two meeting Shotaro worked for another, much more hard-boiled detective whose death helps to kick off the events of the series.
Emerson Cod from Pushing Daisies. He would like nothing better than to be able to just have Ned get the pertinent facts from a corpse, and then collect the rewards in short-order, with as little effort for him as possible. He also primarily deals in cases where the death has been written off as an accident or the police themselves offer a reward for any valuable information related to a case.
Gossip Girls Chuck Bass has one on speed dial. And that's not his only private investigator. His father also had a couple.
This is a decent description of Michael Westen from Burn Notice when he's working for his Client of the Week. He's even used private eye as a cover ID at least once, though on another occasion he notes that a PI license has privacy concerns (anything they uncover could end up in court).
Adrian Monk of Monk used to be police detective, but after his wife's murder he resigned and became a private detective that helps the police.
The title of Psych refers to the name of the "private, physic detective agency" main characters Shawn and Gus run. They often get hired as offical police consultants as well, however.
Yours Truly,Johnny Dollar
Richard Diamond,Private Detective
In a homage to classical noir, Dino Attack RPG brought in a Private Detective actually named Bogart when it became clear that an unknown figure was conducting a series of murders.
Private Investigations by Mark Knopfler of Dire Straits.
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.
Attention-seeking "Ace Detective" Luke Atmey in Ace Attorney, who is always just a step behind Phantom Thief Mask*DeMasque. Step ahead, actually. He's the one planning the heists and sending the thief the plans anonymously, so he both gets the items and makes himself look good.
Despite being a defense attorney, Gregory Edgeworth is dressed like a stereotypical private detective in Ace Attorney Investigations 2.
Johnny Garland is one in Shadow Hearts From the New World. Surprisingly; he implies that a lot of the cases are simply helping someone track down missing pets, and takes a case offered by Gilbert simply because he wants more excitement in his life.
Reality On The Norm has the recurring character Max Griff, who styles himself as a classic Noir private eye and has constant financial problems.
In the online game Sleuth, you create and play one who can have a background you either design from the ground up, or you can choose preset backgrounds including an ex-detective disillusioned by the corruption in the system, a freelance reporter, a retired lawyer, or a reformed burglar, among others. Naturally, which background you pick affects which skills you begin with and how you approach the game.
Decoder Ring Theatre's Black Jack Justice is played straight, although there are two detectives, one male (Jack Justice) and one female (Trixie Dixon, Girl Detective), who take turns providing monologue.