An area of strip joints, brothels and other seedy locales. Very much based on reality.
Generally portrayed in one of three ways in fiction:
The liberal version: Drunk happy people, women waving down from balconies. The protagonist gets to have a good time, or gets made fun of for being unable to. The most friendly take on the district.
The pitiful version: Drunks collapsed in the gutter, battered strung-out prostitutes. The protagonist is probably here on business. Possible location in a Wretched Hive. Very common in cop shows and Antihero comics. You're most likely to see this side of the district in the grey light of the morning.
The hostile version: The drunks will pick a fight with you, the women will rob you or knife you in an alley. Possible location in a Wretched Hive. The protagonist is either lost or hopelessly naive, and will be wishing he'd found version 1 before the end of the night. Also common in cop shows, particularly in the first five minutes when they're showing the soon-to-be victim.
Compare Band of Brothels.
No Real Life Examples, Please!
In one scene in Evangelion 1.0, Shinji wanders past one. He is so lost in his own depression (also his SDAT player) that he's largely oblivious.
The Yoshiwara district of Tokyo is mentioned from time to time in Ooku. Shogun Iemitsu downsizes the Inner Chambers and restocks the decaying pleasure district with desirable men after a gendercide plague destroys the market for female prostitutes and leaves only dregs of men. The Reverend Kasuga earlier hired a few women from there for a plot.
Downplayed in Heat Guy J. Kabuki Road does have (or is implied to have) brothels, strip clubs, etc. but there is other entertainment, too.
Total Recall: Venusville in Sector G on Mars. Features "The Last Resort" bordello and psychic mutant children.
Literature
The Red Light District of New Orleans in the early 1900s is heavily featured in The Meq, though it doesn't really fit into any of the above three categories because it's shown so realistically.
The city of Ankh-Morpork in the Discworld books has an area known as the Street of Negotiable Affection, which is maintained by the Seamstress's Guild.
Renamed from "The Whore Pits" after much representation from said Guild.
Huntsdale, the city Wicked Lovely is set in. Aislinn apparently prefers in to the bright, shiny, happy places because it has more iron, and so keeps The Fair Folk away....except when it doesn't.
George Alec Effinger's Marid Audran series takes place in the Budayeen, a 23rd century Red Light District in a Cyberpunk Muslim/Arab city.
The caravanserai area of Vorbarr Sultana in Barrayar. By the time of the later books in the series it has been cleaned up and gentrified (but you can still get into trouble in some of its back alleys.)
The city of Sharn in Eberron has three red light districts — a tacky and tawdry one, a middle-of-the-road one with the biggest changeling influence, and an arguably tasteful one.
In Exalted, the Demon City has a gold light district. This is because gold is seen as an obscene color in hell due to its association with the sun. Also, the district is a very powerful demon in her own right.
Most such locations in Exalted are blue light districts, the colour chosen to honour the Maiden (ahem!) of Serenity. In fact, the Maiden of Serenity's office is one of these, as it's modeled after the first whorehouse in Creation and still serves as a bar, whorehouse and opium den while also serving as a divine government building.
The musical Tenderloin features a historical example from the Big Applesauce.
The musical Les Misérables features the song Lovely Ladies, which involves prostitutes singing about their occupation and is set in what is meant to be one of these. Starts off as the "happy" version and gradually segues into the "pitiful" version.
Dragon Age II: Kirkwall has the fantasy equivalent of this, the "Red Lantern District". It contains the Blooming Rose Brothel. Therein lies a apostate mage (or as Isabella calls her, an 'Apostitute') that uses mind control on Templars.
FFVII's Wall Market in Midgar is mostly the liberal version, with a dash of hostile. It's a lot more inviting than the rest of the slums, and the girls at the local sex hotel don't seem like they're suffering from much psychological damage (the player even gets to read what they're thinking). However, the district is run by a crime lord, and it's heavily implied that women get kidnapped and forced into prostitution on a regular basis. On your first visit, one of the NPCs even suggests that you sell Aeris off to the Don.
Raidou Kuzunoha vs. The Soulless Army has one. It's important all of one time, then it's only good for dialogue collecting. Oh, and some careless person left a whole clip of fire bullets in front of the giant lamp.
The Futurama video game has a level in the Red Light District.
Would you believe there's one of these in Professor Layton and the Diabolical Box? Layton and Luke catch the owner of the Molentary Express, Mr Beluga, walking into a cabaret and attempt to converse with the woman standing outside. Not only is she one of the show-runners, but she hits on Luke (who's only twelve) after hitting on Layton.
Duke Nukem 3D Episode 1 Level 2 is called Red Light District. It contains a porn shop complete with private video booths, a bar, and a strip club.
Left 4 Dead 2's DLC The Passing has some Red Light influence, in fact, the song that plays during the intro is called Dead Light District
Circuits Edge, the 1989 CRPG, was based on George Alec Effinger's Marid Audran novels (see Literature, above), and like them is set in the Budayeen, a Red Light District.
In an episode of The Simpsons, Homer is asked to take the dog to the vet to be neutered (after the dog had fathered a litter of puppies in an episode parodying 101Dalmatians), but he feels bad for Santa's Little Helper. So Homer takes SLH to the Red Light District of Springfield, which has not only porn theatres, prostitutes, etc. for the human men, but also for dogs to enjoy. Homer even pays to play air hockey with a prostitute while SLH has some "fun" with her dog.
