The lake in Middle Park resembles the shape of an alien. This can easily be noticed on the map or from the air in a helicopter.
In the Broker Safehouse, there is a picture of Roman and an unnamed woman. Roman looks completely different, appearing to be quite lean and muscular. The image appears to be a real photo with Roman's and the unnamed woman's head placed over the top of the original people's faces. The source of the photo is unknown, but it may belong to one of Rockstar's developers. This may be a reference how Roman has lied to Niko about everything.
If the night sky is inspected with a sniper rifle on a clear night, galaxies and stars along with and a flying thumb, in the thumbs up position, can be spotted.
Within the Statue of Happiness is a giant beating heart, supposedly "The Heart of Liberty City." In GTA IV, access into the area is only achievable via an entrance marked by a "No Hidden Content This Way" sign at the upper level of the the statue's pedestal, which can only be reached by helicopter. Once inside the pedestal, the player must scale a ladder up to the midsection of the statue, where the heart is located. It can be shot at, but nothing happens. In The Ballad of Gay Tony, if Luis drinks too much, he will re-spawn at random locations, one of which may be the Heart of Liberty City.
All Mavericks and Maverick variants, including the Police Maverick and the Helitours Maverick, contain an unsettling warning message inside the cockpit on the passenger side of the dash that reads "This helicopter might crash but we really don't care if it does, refer to flight manual for help."
While driving the Mr. Tasty, several songs can be triggered to play by clicking the horn. One of these songs is the theme song of GTA IV. While this song is playing, if ZiT is called, they will text back with "ZiT! Spotted GTA IV Theme." This is a cheat that gives the player health, armor and ammo, but also disables some achievements.
At all TW@ internet cafes, behind the cashier's desk, and also inside various vehicle dealerships, the screen of the Kakagawa Power Ultra 3000 Pro photocopier reads "ERROR Unleash ninja guru lol." This can only be seen through the scope of a Sniper Rifle.
During friendship activites with Packie McReary, if the camera is positioned properly inside his head, a box with texture of balaclava on it can be seen inside his head. It is the texture used for Packie's balaclava in the mission Three Leaf Clover.
During an Online Multiplayer Match, some of the TVs in safehouses and the TV in the MeTV building show the game's pause menu without the map.
One of the restaurants is called "Viendemorte," which sounds exactly like "viande morte." It means "dead meat" in French and is run by a man called Headlamp who never constantly debates on why he is right.
An office in Algonquin called GetaLife is pronounced like "get a life", an idiomatic phrase.
References to the GTA series
In one of the in-game commercials, there is policeman named James O'Toole, who may be reference to JD O'Toole, who was present in GTA LCS.
There is a Liberty Tree article that talks about urban myths in Liberty City. This could be referring to the myths that were created around San Andreas.
References to other video games
A pedestrian bearing an uncanny resemblance to Jimmy Hopkins, the protagonist of another Rockstar's game Bully, can be seen roaming the streets of Liberty City. He is even wearing a Bullworth Academy vest. He is, however, much more aged but does have a similar buzz cut hair style.
In the Lure Dealer's house, there is a crowbar from the Half-Life series.
On the TV channel Weazel, there is an animated TV show called Republican Space Rangers, which spoofs the Halo series, as it features space marines wearing a green armor that resembles the MJOLNIR armor, as seen in Halo. In one scene, the Rangers fly their spaceship through a ring that resembles a HALO ring.
Certain random female NPCs will, upon being pushed, exclaim "Well excuse me dorkwad!" It is most likely a reference to the Legend of Zelda cartoon.
In Firefly Island, there are blue Hippo trash bins, just like the ones from Manhunt.
Numerous references are made to Carcer City, the setting of Manhunt.
The achievement/trophy for completing all the most wanted side missions is called Manhunt.
On the website Yourmexicandoctor.com, two drugs first seen in Manhunt 2 (Hingmyralgan and H4PP1) are shown to be for sale.
On the wall in the arcade place in Chinatown, there are several sprites on the wall resembling those of old games. One of the sprites is a blonde man, with a long mustache and plumber-like black clothes. The sprite resembles Super Mario from the Super Mario Brothers series.
In Cerveza Heights, Dukes, a factory called Paint One Building is covered with artworks of previous Grand Theft Auto games. These includes one of the twins in Grand Theft Auto: Vice City, El Burro and a member of the South Side Hoods from Grand Theft Auto III, OG Loc from Grand Theft Auto: San Andreas and more.
References to other media
On the streets of Liberty City in Algonquin, a pedestrian on the phone can be overheard saying "Hello, it's your friendly neighborhood Super-Star." This phrase is identical to Spider-Man's greeting. Super-Star is also a parody of Spider-Man.
If the one goes on a helicopter ride with Brucie, and if he orders the one to go to the Rotterdam Tower, Brucie might say "If I was that big chimp, I would climb it too." This is a reference to King Kong.
At the beginning of Clean Getaway, Vlad is nearly hit by a taxi and shouts "Hey! I'm walking here!" in reference to the movie Midnight Cowboy.
When the one wounds a cop, sometimes he will shout "I'm gonna be okay!" It is a reference to Reservoir Dogs, when Mr. Orange is shot in the abdomen and is screaming in delirium.
The mission called I Need Your Clothes, Your Boots and Your Motorcycle is a reference to Terminator 2: Judgment Day, in which Arnold Schwarzenegger says this famous line.
The mission Weekend at Florian's is a reference to the comedy movie Weekend at Bernie's Moreover, Florian insists that his new name is Bernie.
In the mission Final Interview, if the one shoots the guard standing at the top of the last flight of stairs when escaping from Goldberg, Ligner & Shyster, he will do the Wilhelm Scream as he falls down.
References to the real world
An online article on Liberty Tree's website, about an Air Sol flight, says it has 212 passengers on board. 212 a reference to the area code in New York City.
In Willis, there is a shop called Al's Furniture. This is a reference to Italian-American gangster Al Capone, who was officially the owner of a furniture shop.
On the stairway leading to the sea at Meadow Hills, there is a "Banksy-esque" style graffiti piece.
In Cluckin' Bell restaurants, near the back of the store, there's an action figure of a man in a martial arts position wearing a chicken hat. Printed on the glass is "Cluck Norris," a reference to film/television star and martial artist Chuck Norris.
In the second apartment the one unlocks, located in Bohan, there are books sitting on a table near the TV. If the player shoots one of the books, there is another underneath that says it was written by Nuck Chorris. This is also a reference to Chuck Norris.
On many alley walls there is graffiti that says "Cope." Cope is a famous graffiti artist in the real world.
The game references to the economic recession faced by the United States on a few occasions, mostly through the in-game internet and news reports on the radio.
The fact that a secretive government body operates out of a paper factory may be a reference to the TV show Heroes. A secretive organization in that show also uses a paper factory as a cover and much of the action takes place in New York.
Also, in the mission Final Interview, the lawyer Tom Goldberg, which Niko has to kill on orders of Francis McReary, is supposedly based on a real-life disbarred lawyer, Jack Thompson, who is an activist against violent video games for minors and is a well noble arch-enemy of Rockstar. One of the lines Goldberg says when Niko pulls out a gun during the interview is a real-life reference to one of Thompson's famous lines against violent shooting games that are "murder simulators," which is "Guns don't kill people, video games do!"
There are a lot of people complaining that they have to smoke outside. This is a reference to New York City, where the one can't smoke inside either.
An office in Algonquin called GetaLife is a parody of MetLife, a real life company.
The Dinka Hakumai sedan is based on a wide selection of Honda vehicles. Its name translates from Japanese to rice, referencing the term used to describe (often tacky) cosmetic car modifications that do not increase performance, of which Hondas are common victims.
Avenues running north-to-south (vertically) in Algonquin are named alphabetically after American cities. "A" is at the eastern side of Algonquin; "G" at the western. The names, in order, are Albany, Bismarck, Columbus, Denver, Exeter, Frankfort, Galveston. Denver and Exeter run concurrently ("Denver-Exeter Ave.") south of Middle Park into Star Junction.
Streets running east-to-west (horizontally) in Algonquin are named alphabetically after different types of minerals. "A" is at the southern end of Algonquin; "X" at the northern end. The names in order are Amethyst, Barium, Calcium, Diamond, Emerald, Feldspar, Garnet, Hematite, Iron, Jade, Kunzite, Lorimar, Manganese, Nickel, Obsidian, Pyrite, Quartz, Ruby, Silicon, Topaz, Uranium, Vauxite, Wardite, Xenotime.
Many street names in Bohan are named after prisons - Leavenworth Ave, Alcatraz Ave, Sing Sing Ave, Guantanamo Ave, Attica Ave, San Quentin Ave, Folsom Way (best known from "Folsom Prison Blues" by Johnny Cash,) Rykers Ave (named after the Rikers Island prison on the East River of New York City,) and Joliet Street (Joliet, Illinois.)
In South Alderney, the street names reflect (mostly antiquated) military terms - Musketeer, Grenadier, Chariot, Phalanx etc.
In Broker, roads running north-to-south are named after Native American tribes - Iroquois, Oneida, Mohawk, Onondaga etc.
In Broker there are the street names "Cassidy" and "Sundance," a reference to the outlaws, Butch Cassidy and the Sundance Kid.
The Beatles: Broker contains the streets "Ringo St" and "Starr St," both reference to the Beatles member Ringo Starr. In Dukes there's a Harrison St., probably a reference to George Harrison, and the northwest section of Liberty City contains the Long John Ave, name that John Lennon used at the beginning of his career. Paul McCartney references are not found yet. Also on the side of buses, there is a small ribbon saying "here, there and everywhere" which is a reference to the Beatles song "Here, there and everywhere."
In Alderney, some streets are named after cult leaders: Hubbard St., Applewhite St., Koresh Square.
Some streets are named after tools - Jackhammer, Plumbbob, Tinderbox, Drill.
Some streets are named after insects - Caterpillar, Worm, Butterfly.
Some streets are named after foods - Onion, Asparagus, Chive.
Some streets are named after colonial Settlements - Yorktown, Delaware.
Some streets are named after famous surnames - Harrison, Hancock, Livingston.
Some streets are named after nuclear tests - Plumbbob, Emery, Niblick, Ivy, Hardtack, and Trinity.
Some streets are named after inventors - Edison, Schneider, Moog, Drebbel, Farnsworth, Cockerell.
The full name of street in Northwood, which is named Astoria, is Astoria Place. This could possibly be a take on Astor Place in the Lower East Side.
On the west side of Happiness Island, there is a random spawn that can be a car. This is unusual as there is no way a vehicle can be brought on to the island unless unloaded by a ship.
Behind an old, rundown mansion in Westdyke, Alderney at the end of the dirt roads, there is the hidden, extremely rare Sultan RS parked in some bushes. This is practically the only way to find the car in-game. The only other way to obtain it is if the game has it stored in the temporary memory, when one appears in one of Brucie's Races, and automatically spawns it parked or driving around the streets.
Grand Theft Auto IV: Episodes from Liberty City
After the mission In The Crosshairs in TBoGT, the one can find the Cook's decapitated head on the roof where Luis is attacked.
References to other media
In The Lost and Damned, the dash of the Rhapsody has a sticker of Wayne and Garth from the movie Wayne's World, which famously features the song Bohemian Rhapsody.