Easter eggs
From WoWWiki
An Easter egg is an undocumented, obscure, or otherwise "hidden" command — usually buried deep within a program's source-code and only triggered by executing a certain series of actions or system commands. Often, an Easter egg will reveal an inside joke, though Easter eggs can also be placed into in-game props and scenery or only in areas visible to those that are actively using "cheat codes" (or otherwise circumventing the "normal" operation of the software).
While Easter eggs were once included in virtually all computer software, software companies such as Microsoft have discontinued the practice, citing security concerns associated with undocumented (and often untested) code.
For more hidden in-jokes and references, see in-jokes and references.
Easter eggs in-game
These are mostly Blizzard-related Easter eggs.
See In-jokes and references for more.
Sega!
Female gnomes shout "Sega!" when they die, a reference to Sega television ads where someone would shout it.
Guards
Guards of cities will respond to some emotes such as /wave, /rude, /kiss, or /salute. This does not apply to all guards though, most notably Darnassus Sentinels do not reply to emotes and even the same guard will not always respond. However, if you ask a Darnassus Sentinel about the enchanting trainer, she may respond as if you said she was enchanting. Also, if a blood elf paladin /eye a Silvermoon Guard, they will /kneel.
Popeye
Ol' Beasley in the Old Town district of Stormwind City says, "I will gladly pay you Tuesday for a hamburger today.", an obvious reference to Wimpy from the Popeye cartoon.
Goblin Shredder display
If you look into the cockpit of a Goblin Shredder, you can see a picture of 
Kerrigan from Starcraft on the control panel.
Michel Koiter
At the Shrine of the Fallen Warrior, there is a memorial dedicated to an orc with the initials "M-K". M K was Michel Koiter, one half of Twincruiser, a Blizzard illustrator who passed away during the development of World of Warcraft. If you visit the memorial, you will find a spirit healer named Koiter. Most likely named in dedication for the once beloved man.
Mountaineer Pebblebitty
Players who are fed up with the rude female dwarf who presides over the quest chain to obtain the key to Searing Gorge, Mountaineer Pebblebitty in extreme southern Loch Modan, may feel inclined to use /rude on her after all the trouble she puts them through. Blizzard has anticipated this and provided an appropriate response.
Deeprun Tram sightings
When you are riding on the tram, when in the underwater area, you can sometimes see Nessy, a naga siren, a Deeprun Diver, a Basking Shark, or a Deeprun Chest.
Prairie Chicken pet
In Westfall, you can find a Prairie Chicken small pet egg quest at Saldean's Farm. In order to activate the quest, you need to do the emote /chicken to one of the chickens at the farm. You can put the emote in a macro to make it easier and faster. Prior to patch 3.0.9, this quest is only available to Alliance players, although it is possible for a Horde player to acquire a chicken with Alliance aid. Since patch 3.1.0, it is now possible for both sides to complete this quest.
Vanity pets
If you own a non-combat pet and stop moving for a while, it will begin to sleep or do other kinds of emotes.
- The Zergling pet will chomp at air.
- The Murky pet will sing and dance.
- The Mini-Diablo pet spouts out fire when he is idle.
- The Panda Cub and Sleepy Willy lie down and sleep (you see green Z Z Z ...) when idle.
- The Speedy pet will shout, scaring nearby critters.
- The Peanut pet will occasionally blow his nose, making a noticeable elephant noise.
- The Willy pet will occasionally kill nearby critters with an eye-beam.
- The Egbert pet will run around like a caffeinated two year old.
Old Teamster's Skull
While fishing, you have a chance to obtain an "[Old Teamster's Skull]", a likely reference to teamster unions and their boss Jimmy Hoffa. One day, he mysteriously disappeared and his body was never found. There is a description on the item that says, "Looks like someone didn't like this guy." as flavor text.
Stratholme Spectral Citizens
If you type /dance on the civilian ghosts in Stratholme, they will respond in kind and dance with you. Even if you attack them, subsequently dancing with them will make them dance with you instead of continuing to attack you.
Also, appropriately, they will respond to the /rude emote with a rude action of their own.
Drysc In Exodar
There is a draenei walking around in Exodar by the name of Drysc, a WoW CM.
Eyonix In Stormspire
The innkeeper of the Stormspire in Netherstorm is named Eyonix, a WoW CM.
Raid encounters
An NPC named Auctioneer Naxxremis in Undercity references Naxxramas, fortress of Kel'Thuzad, while "Cookie" McWeaksauce in Odesyus' Landing has a parrot named Ragnarose, referencing Ragnaros, boss of the Molten Core. Also, when everyone journeys to Blood Watch to celebrate the hero of Bloodmyst and Azuremyst, there is an elekk next to Kessel named Archimindy.
O RLY? YA RLY!
One of the auctioneers in Booty Bay is named O'reely, while another in the Undercity is named Yarly. This is referring to the common internet phrase "O RLY?" and the reply "YA RLY". This is widely personified by a snowy owl and a great horned owl. Also, there is an NPC owl in the game named O'Reilly.
Potent potables in Stormspire
In Stormspire, Dealer Sadaqat is a vendor of "Potent Potables" (potions). The game show Jeopardy! uses this catchy phrase as a category with unusual frequency, leading to its becoming an oft-reused comedy meme, including in Saturday Night Live's "Celebrity Jeopardy" skits.
Hindenburg
At the zeppelin tower at Tirisfal Glades is a zeppelin master called Hin Denburg — this is a reference to the crashed German airship called LZ 129 Hindenburg.
Klatu, verata, nichto!
In Honor Hold's inn, on the second floor, you can find Assistant Klatu. He assists Anchorite Barada in dispelling the ghosts that haunt Colonel Jules. This is a reference to Sam Raimi's movie Army of Darkness in the scene where Bruce Campbell (Ash) has to say a series of words correctly before removing the Necronomicon from its stand. Ash forgets to say "nichto" and instead mumbles an incoherent word. This was itself a reference to the 1951 film The Day the Earth Stood Still in which the phrase "klaatu barada nikto" is used to prevent the robot Gort from destroying Earth after the alien Klaatu was injured. Additionally, Anchorite Barada, also in the inn, is a direct reference to The Day the Earth Stood Still.
Hidden Mickey Mouse
An exceptionally well hidden Mickey Mouse is located in the Valley of Honor within the Horde city of Orgrimmar. It is so large, and so well sculpted into the surroundings, that most people will never know that it is there. The only way to see it is to manually increase the maximum zoom setting of the camera.
Fun graphics
Along the Webwinder Path, there are some felled trees. One end has a large smilie face on it.
There is also a large smiley face underneath Karazhan. You can see it if you log out near Karazhan. Once you log back in, before the doodads and NPCs load up, you can see the smiley face. Humans can get under Karazhan to the smilie face by jumping to a certain ditch in the corner of the rubble in the back side of Karazhan (facing the crypt/graveyard).
In Searing Gorge, if you head just a little north of Blackchar Cave and zoom your mini-compass all the way out, you can see a smiley face in it.
When floating underwater, one of the motes that float by will very occasionally be a tiny smiley face.
In Stranglethorn Vale, the vines on trees look like orc faces, or possibly, the Great Deku Tree from Zelda.
Snakes on a plane
At the Nesingwary Safari, on Hemet Nesingwary's crashed plane, there are level 1 critters on the plane, snakes. This is an obvious reference to the movie Snakes on a Plane.
The Fitz
Also at Nesingwary's Safari in Nagrand is Shado 'Fitz' Farstrider, a possible reference to various characters (Chade, FitzChivalry, and the Farseer house) from Robin Hobb's Farseer trilogy.
They're coming right for us!
Again at Nesingwary's Safari, Hemet will sometimes be randomly attacked by talbuks. When he is attacked, he yells out, "Look out, Fitz, they're coming right for us!", which is a reference to a South Park episode, "Volcano"
Florist Gump
In Stormwind City, there is an NPC selling flowers by the name of Bernard Gump. An obvious reference to the film Forrest Gump as his title is shown as "Bernard Gump <Florist>"
Shauly Pore
In one of the ecodomes in Netherstorm, there is a goblin named Shauly Pore, a reference to Pauly Shore in the movie Bio-Dome.
Junk Loot
Some of the zombie NPCs in Duskwood drop Pound of Flesh — a reference to William Shakespeare's play The Merchant of Venice.
The apes in Ungoro Crater drop Empty Barrels — two possible references — "a barrel of monkeys" or possibly Donkey Kong.
Various NPCs drop Shark Bait, an item which is a reference to the movie Finding Nemo. "Shark Bait" is the nickname Nemo's fellow fish tank mates gave to him. The flavor text reads "Just keep swimming..." which is a quote from the blue fish, Dory. The icon is a clownfish like Nemo himself.
Pink Floyd
When performing the Horde 
[45] Triage quest, one of the injured soldiers may say, "Goodbye, cruel world... I'm leaving you today... Goodbye... goodbye... goodbye.", a reference to the song "Goodbye Cruel World" from Pink Floyd's 1979 album, The Wall.
Otonambusi, the general goods vendor at Azure Watch, occasionally says, "If you don't eat your fish, you can't have any pudding," and "How can you have any pudding if you don't eat your fish?" These reference the school master in the song "Another Brick in the Wall" from the same album, who says, "If you don't eat your meat, you can't have any pudding," and "How can you have any pudding if you don't eat your meat?"
The Horde NPC Floyd Pinkus at Thrallmar in Hellfire Peninsula is another obvious reference to Pink Floyd.
Mario and Luigi
The items Essence Infused Mushroom and Power Infused Mushroom give 200 health/mana; Goombas in the Mario games give 200 points when you jump on them.
Larion and Muigin, when you get rid of the N's and switch around the first letters, appear as Luigi and Mario. They also wear red/green suits and hammers.
Ringo
NPC Ringo found at [51, 50] is an obvious reference to the Beatles drummer Ringo Starr. Furthermore, the quest A Little Help From My Friends is taken from the title of the Sgt. Pepper's Lonley Hearts Club Band track With a Little Help From My Friends. This is one of the few tracks Ringo sang lead on in the Beatles discography.
Teenage Mutant Ninja Turtles
In the Dalaran Underbelly, there are four small Dalaran Sewer Turtles surrounding a large Dalaran Sewer Rat. The turtles don't move, as if listening to the rat, who is bobbing up and down like he is speaking to them. It makes one think of Master Splinter instructing the TMNT.
Winterfin Tadpoles
The tadpoles you need to rescue in Oh Noes, the Tadpoles! act significantly different if you attempt to open a cage when you are not on the quest. Also, if you are not on the quest and try to open the cages, they will open slightly but then get closed quickly by the tadpoles.
Sega CD
In the Dalaran Underbelly, a shark can be found named Segacedi. The is a reference to the Sega CD and to a Sega CD game, Sewer Shark.
ED-210
In the Borean Tundra, there are two quests that involve killing a machine named ED-210: The Sub-Chieftains for Horde players and Lefty Loosey, Righty Tighty for Alliance players. This name could be a reference to ED-209 from the Robocop series.
Wolverine
In Dragonblight, north of Moa'ki Harbor (in the area with the Snowfall Glade Pups for the Kaluak daily), is a wolvar named 'Loguhn' with a set of three bladed fist weapons. An apparent nod to the X-Men superhero Wolverine — who is most commonly known as Logan among his fellows and has similar claws.
Final Fantasy 7
The Dalaran flower girl is called Aerith Primrose, this referes to the final fantasy 7 character Aerith Gainsborough who is referred to as the flower girl at the start of the game.
Lost
There is a small island in the Bittertide Lake in Sholazar Basin with a door and a handle, on which there is a plaque that reads "5 9 16 17 24 43". It's a reference to the sequence of numbers "4 8 15 16 23 42" from the show Lost.
Help
Northeast of Thunder Bluff on top of a mountain is a line of text that reads "HERB", but at another look you will see that it says "HELP" backwards.
H1N1
In Borean Tundra the pigs near the horde town will sometimes infect you with Swine Flu, there's a chance on hit to spread the flu to other players. (This was in the game well before the media coverage of H1N1).
Zelda
There is a quest in Un' Goro Crater where you obtain a sword that looks similar to the Master Sword and a boomerang after you complete it, both references to Zelda. The gnome who is the quest giver also has link in his name, also a reference to Zelda.
Please update this article with more specifics, it's been a while since I did this quest and I don't know all of the details.
The Starting Quest leading to it http://thottbot.com/q3844
