This is the character in Cop Shows, Detective Dramas, and Espionage Thrillers that everyone despises. Even the cops who make use of his services hate him. He is beneath the level of more dignified, "honest" crooks. He is a low level version of the Knowledge Broker; perhaps even the minion of one. But where a Knowledge Broker is high status and has an arbitrarily greater reputation, this character is just a stoolie. (This difference is sort of like the difference between a courtesan and a street-walker... they are both prostitutes, but one is considered "high class".)
The Informant lives in a miserable fashion selling information. But he is needed, so he is tolerated. Sometimes a sort of patronizing affection is granted him. In such a case, he is likely to be a Lovable Coward as well as a Lovable Traitor. Probably there are those who will not find him lovable...
A subtrope of Knowledge Broker. Mysterious Informant is a Sister Trope to, and The Stool Pigeon is a Sub-Trope of, The Informant. This trope may be averted entirely if a particular crime boss or criminal empire is considered too scary to inform on.
Not to be confused with 2009 Matt Damon film The Informant!, which is actually about a Stool Pigeon, kind of.
The informant in The French Connection is portrayed more seemingly realistically than the norm described in the trope description at the top of this page, perhaps because the film is based somewhat closely on a true story.
In Guy Ritchie movie Rock N Rolla, the London underworld is puzzled by he identity of an informant whose testimony has been putting away multiple well connected gangsters. At the very end of the film it's revealed to be The Don himself, Lenny. Lenny's men quickly and ruthlessly turn on him, especially since some of them, including The Dragon have gone to prison based on that testimony.
In Hellraiser: Inferno, Detective Joseph has an ex-con snitch named Bernie running an icecream truck, whom he abuses for information on the Engineer. Then again, Bernie is implied to have been sent to prison for being a pedophile. The Engineer tortures and murders the informant on camera to spite Joseph.
Nick the Nose from Donald Sobel's Two Minute Mysteries series.
In Discworld, Lord Vetinari gets intel from the city's beggars, mostly by way of unfounded rumors that he pays for information. And the City Watch under Commander Vimes gets information from similar sources, as well as some of the pettier of petty criminals, who have an almost jocular relationship with the lawmen. It's mentioned that the Watch House break room is populated by a collection of "petty crooks, informants and ex-watchmen, some of whom were the same people." Vimes signs off on the doughnuts, since the information they bring in is well worth the price.
In Night Watch, Nobby Nobbs started out as one of these, and eventually became a real member of the Watch. Though not a particularly good one.
Live Action TV
Willy the Snitch from Buffy runs a dive patronised by demons, and lives up to his name
In Miami Vice, Crockett and Tubbs regularly called upon Izzy "The Snitch" Moreno and Neville "Noogie" Lamont to get information on whatever big plot was going down. The two cops treated the latter like some unpleasant form of fungus, while the former was treated like a favored pet.
In Plain Sight, by its very nature, often deals with Informants who are now on the run from their former employers.
While very few of the Informants were ever actually shown, the detectives on Law & Order often had regular informants they relied on. Detective Rey Curtis often referred to an entire network of confidential informants he had from his days as a vice cop.
In the episode of Fantasy Island with the two old actors who used to star in a cop show together, their characters had an informant they used to rough up when they wanted information. When they get stuck in a real-life version of the show, the informant shows up with a massive grudge against them for the way they used to treat him.
Alexei, a Russian taxi driver in Moscow, in JAG a Lovable Rogue that works for "the highest bidder", but develops a crush on Mac (doesn't everyone?}.
In the fourth season, Randy's story is used to show the darker side of this trope and illustrate how becoming a police informant can completely ruin your life, even if you're in middle school.
Several one-off characters in The Sweeney, whose portrayals ranged from "contemptible but necessary" to actually fairly sympathetic. Of course, one of the show's central themes was that the cops and the cons were Not So Different.
In the Jonny Quest TOS episode "Terror Island", Race Bannon's "old friend" Jade questions an unnamed informant, who pulls the old "more money might help me remember" trick until Jade gets tired of him and pulls a gun.
Josh Stone from Recess was closer to this trope than The Stool Pigeon, as not only was he undercover for the Superintendent Skinner, he was also not even a kid. He did such a good job at covering his tracks that TJ and his friends initially thought Randall was behind their being snitched on, despite Randall's insistence that he did not, until they literally chased him into a bathroom.