All of the Sam & Max easter eggs in the Monkey Island series. I am incredibly astonished that there has been a reference in every game thus far(fingers crossed the freelance police find their way into the new game)

Saints Row 2: Plesiousaur/Nessie. I've always been a fan of Nessie, so it was an absolute delight for my teenage self to find a little island off the coast of Stilwater that had the skeletal remains of a Nessie-like creature. I believe it also had a cave with a statue of the Volition logo in it as well. It's the little things in life.

All the easter eggs from Torchlight 2.

Especially the one from Silence of the Lambs. The Buffalo Bill's Hide Mask!

I remember being blone away when I first found the heart inside Lady Liberty in GTA IV. I just never expected something this ridiculous to be hidden in such plain sight

In Across the Obelisk one of the starter characters named Magnus is a wolf and if you hover for a second or two over him with your mouse as if you would pet him he replies either with something like "Stop petting me" or "Give me more"

DUKE 3D Now that's one DOOMED space marine.
https://i.imgur.com/2lHHqbt.png

*Metal Gear Solid 3 Spoilers*

My favorite secret / easter egg is found in Metal Gear Solid 3. I stopped playing Metal Gear Solid 3: Subsistence for a week just as I got into the fight with The End, when I booted the game he died of old age.

Now, I've seen people mention the stump easter egg in Curse of Monkey Island and the fact that you can die in the original...

But, did you know...

On Blood Island, if you keep insisting on getting water on the beach (about 25 times), Guybrush will actually enter the ocean, and end up back at the infamous underwater scene in the original, complete with a drowned Guybrush. And you can actually go back after you leave the cutscene to explore, by which you'll find that the corpse mentioned earlier is described as 'fish food'.

In the Sonic the Fighters PS3/Xbox port, you can select 3 hidden fighters by using button combos during the fighter select screen,2 of them are Metal Sonic and Eggman which were boss fights in the Arcade mode, but one of the fighters was a scrapped fighter called Honey who was made way back when the game was in arcades as a tie to the game fighting vipers, they added her in for the port as an easter egg which is neat!

Futurama the game where the Easter egg is an Easter egg

Yet another Metal Gear easter egg, and another one I'm sad to see gone from newer versions.

In Metal Gear Solid 3, after you've been captured and tortured, if you save the game, exit and then go back to it, you get a DMC-like nightmare sequence in which you kill strange creatures for a couple of minutes. After that, Naked Snake wakes up, and if you call Sigint, he starts talking about a nightmare he once had about everything turning to sh*t.

Unfortunately, the HD remaster versions lack this easter egg, so if someone wants to experience it, go grab the Subsistence version, since the original Snake Eater only had the top down camera.

In the fan-made remake (and expansion) of King’s Quest 2: Romancing the Stones, there’s a section of the game where King Graham is doing some kind of vision quest, and he can see his possible futures.

The player can pick among several choices and see where they will lead the king in his lifetime.

One of the final choices, called “King Graham’s Madness,” will see a very old King Graham in his throne room with a watching crowd, passing off his crown to a young knight—but not without first demanding the knight slay a most dreadful, hated beast.

Graham only describes it in ghastly imagery; the knight makes a few guesses but gives up when he can’t figure out which creature he is supposed to defeat, and he asks what it could be.

Then the creature enters the room.

“Hooooo! Sorry I’m late, Graham!”

“That,” the aging king growls through his teeth.

And then he charges at Cedric the Owl with a sword.

Anyone who played King’s Quest V knew the pain and recognized him immediately.

I laughed so hard, and it still brings a chuckle when I remember it!

Id have to say easter eggs i have been the biggest fan of are the ones in portal/porrtal 2, for example the half life references, with gordons crowbar or all the vague references thats become easter eggs in different games, really enjoyable experience that

leatherface in GTA! i didnt know about it, but i love it because it was scary and i later found out what it was on youtube

if that Hero 3D terminal in Iji counts, then I'm in.
ADD: also, more a cameo, but in AC and ACPR versions of Guilty Gear XX May Ship stage has Johnny's crew replaced with "Haruhi Suzumiya" and "puella magi Madoka*Magica" casts respectively.