In the very southwestern block of Bedford Point lies a parking lot (notably featured in the Kingdom Come mission) where an easter egg can be located; known as The Hidden Sign. To reach it, the player needs a car of small size such as Manana or Taxi. Going up the stairs leading to an overlooking platform to the north of the lot (near the entrance to an underground parking in the same area), the player must jump from the car to reach the top of a building to the west, jump down on the wall and from there drop down to an otherwise inaccessible alley behind it. At the end of the alley, on the wall, there is a posted notice that states "You weren't supposed to be able to get here you know." Near the poster is a ramp that allows the player to leave again.
If you leave the city limits of Liberty City by using the Dodo and fly straight north for a few minutes, you can see various Rockstar Games' programmers edited to look like town names on the mini-map, e.g. Obbeburgh (Obbe Vermeij), Aaronsville (Aaron Garbut), Woodcunty (Alisdair Wood), Les County (Leslie Benzies), Garystown (Gary McAdam) and Adamton (Adam Fowler or Adam Cochrane). However, these towns don't physically exist within the gameworld, and only their names can be seen on the radar.
Inside of the TW@ Internet Cafe there are computers on the first floor displaying screenshots of GTA 1, and GTA 2.
References to other games
On the in-game radio, you can sometimes hear references to Carcer City, which is a city that would be later used in Rockstar's Manhunt.
In the mission Two-Faced Tanner the character that the player has to kill is named Tanner, protagonist of the Driver series. Humorously, he was given a female walking animation due to the criticism that Tanner in Driver walked like a girl.
Rockstar Games are known for hiding their logo in secret places in their games. There is, however, a poorly disguised Rockstar Games logo on top of a unusable helicopter on the roof of Kenji's Casino. The logo is poorly disguised because it even reads 'Rockstar Games' below it. The logo can also be found on a hat of the pedestrian that wears headphones (and is famous for the mysterious tune heard through them).
In Staunton Island, there is a store with a banner above its enterance. The banner is a Rockstar Games' self advertisement banner as it reads GTA 3 OUT NOW and WWW.ROCKSTARGAMES.COM.
There is a particular construction worker pedestrian that can be found in the game, and when bumped into, he will say segments of the song YMCA by Village People. This is an easter egg, as the lead singer in that song also is a white construction worker. The man is heard speaking various phrases such as "Young Man", "There's no need to be down" and "I'm in the YMCA".
Car stuck in Liberty City underground. This is what looks like a 20th century car getting stuck when the underground was build.
The yellow notebooks carried by male students of Liberty Campus feature scribbled drawings of ejaculating male genitalia.
In a Football Stadium in Stunton Island, the crowds in it are making the word COCKS.