I remember one from FEAR 3, it was a sign with a baby face hanging in a street that read "Cuidado de ancianos. Pase tiempo con sus seres queridos mientras puedas" which translates to "Elderly care. Spend time with your loved ones while you can" (I played the spanish version). Pretty weird to see a sign like that in a shooter, either way its a good message.

The Big Metal Unit, from the Baldur's Gate trilogy.

You could find a unique set of clothing in each entry (including ToB)

Golden Pantaloons
Bronze Pantalettes
Silver Pantaloons

Only the first had a long description, but it was quite amusing. The other two were much more generic sounding, oddly they had seemingly no purpose, however if you imported the same character through the whole series and they were holding these items in their inventory they would carry over through the games.

Later in ToB you could provide all these items to a reclusive mage who would create for you the
BIG
METAL
UNIT

A full plate wearable by any class, that set your AC to -10 (extraordinarily good in 2nd edition), but required fifteen strength to don. It had a long and funny description, it came with a huge custom weapon with several ammunition types.

Wearing it changes your appearance to that of an adamantine golem, including causing trouble trying to go through small doors.

The Big Metal Unit is very silly, strange, hard as heck to get (pieces being hard to find), takes from one of the first areas of the first game to one of the later areas of the second game's expansion pack and was a lovely payoff to the fans who had originally found the Golden Pantaloons three years before the release of the ToB expansion where it finally became possible to complete the set.

In Indiana Jones and the Infernal Machine, the loading screen for the Teotihuacan level (which consists of an iconic Indy-style travel map) marks the locations of Melee Island (from the Monkey Island series) in the Caribbean and the city of El Marrow (from Grim Fandango) in Mexico. I was so happy to recognize these two names that I opened a new thread in the game-specific subforum to share a screenshot. :D

https://www.gog.com/forum/indiana_jones_series/monkey_island_and_grim_fandango_references_in_infernal_machine

Later in that same thread I posted about a couple other Easter eggs referencing The Curse of Monkey Island, but only after reading about them. I must admit I never would have been able to find them on my own!

Super Mario Odyssey has a bunch of those. Costumes being references to other Mario games are cool, but here's a funnier one: when you let Mario stay still for too long, he starts yawning, and then sleeping on the ground. That's one thing. But if you let him stay still for even longer, a bird may come and perch itself on Mario's nose ^^

i liked the photo of elvis presley in the five flaggins inn in baldur's gate 2 shadows of amn

My favorite easter egg is in The Curse of Monkey Island™ where Guybrush finds himself in the Goodsoup family tomb. He sticks his head up through a crack in the crumbling ceiling to discover the world of Monkey Island™ 1 in all its pixel glory.

Speaking of the original's art style, the image editor they used was called Deluxe Paint. The software called sprites "brushes", with the file containing the test art for the main character named "guy brush". Another fun easter egg.

The 79 Steps of Enlightenment - Shadow of the Colossus.
The Decade Long Quest For Shadow of the Colossus’ Last Secret.

I don't need to explain this.
If you know, you know.

Dungeon Keeper - If you play over midnight on New Year's Eve, everyone in the game stops and dances to Disco Inferno.

I say Oshima in Cyberpunk (the Hideo Kojima cameo).

Blood had a pretty good clap back secret to Duke Nukem.

In the map Dark Carnival (E1M4), in the room with the snake pit, if you hit a warning sign, it'll open up a room behind the snake pit itself. In there, you'll find some ammo and a rather mutilated Duke Nukem. Caleb will immediately go "I've got time to play with you" in a callback to the Balls of Steel Easter egg in Duke Nukem. Using Duke's corpse will make it swing back and forth prompting Caleb to go "Oh, shake it, baby!"

Do you like fried mush and nug?

I do not like them Mister Klug
I do not like fried mush and nug

Would you eat them on a rug?
If you eat, you'll get a hug!

I would not eat them on a rug
From you I would not want a hug

Then would you drink them from a jug?
Come on, come on, give them a chug!

I would not drink them from a jug
I'd rather eat a slimy slug

Would you eat them with a bug?
Would you share them with a thug?

I would not share them with a thug
I would not eat them with a bug
Not for a hug
Not on a rug
From a jug
I will not chug
Come on, come on, now mister Klug
Are you on some kind of drug?

Eat them in this hole I dug
Eat them, eat them, don't just shrug

I've had it, had it, Mister Klug!
Down, into that hole you dug
Down with the thug
And the slug
And the bug

Written by Paragon Seuss for The Noladar Anthology of Dwarven Poetry, copy of the book found in the Denerim market

- Dragon Age: Origins

My favourite easter egg (out of many) is finding Duke Nukem strung up and bleeding in the "Dark Carnival" level of Blood. It's just SO on the nose. Especially when you shake him and Caleb goes, "Shake it baby!" and "I don't have time to play with you!" (callback to Duke's "I don't have time to play with myself" line). Love it love it LOVE IT!

But both Blood and Duke Nukem have countless amazing eggs, including Jack Torrance from the Shining frozen to death in the snow (and the line "Hereee's CALEB!")... and then Freddy Krueger's striped clothes and hat in another level.

Don't forget finding the Doom Marine in DN3D ("That's one Doomed space marine!") and the T-800 Terminator that was crushed in machinery.

I could probably go on for an hour alone listing all the eggs in those games.

The A 38 questline in the Witcher 3 Blood and Wine expansion.

A nice easter egg reference to Asterix & Obelix, but it also felt so funny, because so true irl.

i be old school and chouse the god of war easter eggs from the first game

after you beaten the game you can go into a throne room with 2 statues of some bosses (Areas and the Minotaur) and if you destroy them you get a secret message with a bunch of numbers, put them togther and you see its a phone number, call it and Kratos and the director David Jaffe will then talk to you and will congratuate you for finding the easter egg

In Prince of Persia 3D, in the first level, The Prison, going into a certain empty cell, to the far left corner, and clicking the action key a couple of times would make the Prince take a pee.

Everything's going bad for the Prince up until that point, but one must always answer the call of nature!