The arena in the finale of Dark Carnival is conveniently named The Peach-Pit. This may in fact be a reference to the diner in Beverly Hills 90210, in which the diner hangout was named the same. Or to a certain Mangaka who goes by that as their pseudonym, which too is based on the same show.
Some of the graffiti (and one of the achievements) in the first game references the "Zombie Genocider" achievement of Dead Rising which rewarded the player for killing 53,594 zombies. Left 4 Dead tops it by exactly one zombie.
It appears that this has caught on; Prototype has a "Trail of Corpses" achievements, for killing 54,596 infected. Difference being that the Prototype Infected are totally not zombies.
Valve introduced a new twist — the "Littlest Genocide" achievement, which requires you to kill 5,359 zombies on the Crash Course DLC campaign.
The first game takes place in Pennsylvania, starting in Philadelphia and moving outwards from there. George Romero's movies were all set in Pennsylvania.
There are countless shout outs to zombie movies ranging from things Zoey says to a room where you can see the remains of a zombie killed by a lawn mower.
The messy lawnmower scene in Blood Harvest is a shout out to the zombie movie Braindead, which was directed by Peter Jackson.
The campaigns themselves are a homage to the entire zombie movie genre. Each campaign is presented as a movie, with a theatrical poster at the beginning and a credits sequence at the end. All in-game graphics are shown under a subtle film grain filter, and musical cues are used to highlight certain situations. Various zombie movie tropes are played with. And of course, the placement of enemies and items being controlled by an AI Director.
A brilliant reference to a completely non-zombie movie:
The Dead Air campaign features a shout out to Nietzsche. A man spray-painted "God Is Dead" (a phrase made famous by Nietzsche himself) on the walls at several points before he apparently killed himself.
Zoey: "Oh no, the Zombies killed God!"
Francis: "God's dead, huh? Well, join the damn club."
Upon picking up an SMG, Louis might say, "This is just like in Counter-Strike!". Which itself is a reference to how Left 4 Dead started when the dev team of Counter-Strike: Source was messing around with playing as armed Counter-Terrorists against knife wielding Terrorist bots.
Louis has an unused line in Left 4 Dead 2 for picking up a Crowbar. "Man, I feel like Gordon Freeman!"
One of Ellis' community campaign stock lines, "Dude, this is just like Team Fortress 2!"
During Dark Carnival, you may notice "Pyro's Hamburgers" and "Heavy Gulp" signs on some of the stands.
The escape vehicles in Crash Course are clearly modeled on the zombie-smashing buses from the Dawn of the Dead (2004) remake.
Sometimes when Zoey picks up a shotgun, she will say "Groovy."
Usually triggered right after Francis does; he has a similar line:
Francis: Groovy! (either Louis or Zoey snickers audibly) What? It is groovy!
One scene in the trailer for Left 4 Dead 2 shows Coach eating a chocolate bar and another scene has Nick make fun of Coach by trying to convince him to go up the stairs faster saying that the escape helicopter is made out of chocolate. This could be a possible reference to Doc Louis who also had a heavy preference to chocolate.
Note that the chocolate bar Coach is eating was a "chocolate helicopter." Just look on the box by Rochelle which says "choco Heli" and mentions something about blades.
In the first map of The Parish, a jukebox can be played in the bar and has a few songs selected at random, one of them being Still Alive.
"Dark Carnival" may be a shout out to House of the Dead: OVERKILL's "Carny" level, or possibly the "Dark Carnival" level from Blood (a game brimming with shout outs). Or, for that matter, any of a dozen horror movies that have used circuses and theme parks as settings or set pieces.
The gnome you win for getting 750 points at the shooting range game is the exact same model as the gnome from Half-Life 2: Episode 2.
And, just like its predecessor, you get an achievement for carrying it with you for the whole campaign.
Its name, "Gnome Chompsky", references the noted linguist and psychologist Noam Chomsky.
"Swamp Fever" begins the same way "Crash Course" does: one Survivor having shot the infected pilot, and another berating him for it.
The female soldier seen here in Part 2 of the Sacrifice comic may be a shout out to The Pyro as the two share the same distinctive gasmask and suffer from a bit of gender confusion.
Louis has a figure of Heavy Weapons Guy on his desk in Part 1 of the Sacrifice comic.
Part 3 of the Sacrifice comic has a poster of Chuck Greene found in this page. It also has a hidden image of Ellis wearing Chuck's default Case Zero shirt with the text of "The Left 4 Dead 2 team congratulates the Dead Rising 2 team on their release".
At one point in the Crash Course campaign, the survivors must pass through a small area with a broken steam pipe. Francis says "watch out for that steam pipe!", quickly followed by veiled references to a different kind of Steam.