The Metal Gear Solid series have so many fun secrets it's hard to choose, whether it's funny codec conversations or just some funny little detail such as slipping on seagull poop in MGS2.

I'd have to go with MGS1, specifically the fourth-wall breaking by Psycho Mantis, who could read your memory card and comment on the kind of games you play, but also comment on how often you save. And then there's the controller-shaking in the PS1 version. This blew my mind as a kid and it's a shining example of Kojima's genius as a game designer. :)

One classic.
In Wizardry 6 you can read a graffiti "Trebor Sux" Apparently it is refering to Robert Woodhead, one of the original Wizardry programmers. Probably there were little dissagreements of some sort any time before.

Powerslave for the Sega Saturn, finding out about the hidden team dolls, only for them to unlock the game Death Tank when you achieve your goal of finding all of them. Kinda a secret within a set of secrets.

GTA San Andreas. There's this very tall bridge connecting San Fierro to the northwest corner of the map. If somehow you manage to get to the top of it, there's a metal plaque waiting for you there. It reads: "There are no easter eggs up here. Go away.".

Laughed my ass off when I read it!

I think the most interesting secret in the gaming history is just one of the first documented. I'm talking about the Adventure, released for Atari 2600. The Atari programmer Warren Robinett added his own name in a hidden room in the game, because he wasn't credited in it even though he designed the game. But to get to this room wasn't an easy task, as it required to follow a series of painstaking steps.

To get to the hidden room you have to do as follow:

1- Select Game 2 or Game 3.
2- Get the Bridge
3- Find the maze north of the Black Castle. Look for the room that is flickering -- the flicker is due to the extra item hidden in the room related to the Easter egg.
4- In the flickering maze, use the bridge to get to access the chamber in the bottom/middle of the room.
5- Go to the lower right corner of the small chamber and you'll pick up a single pixel.
6- Carry the pixel to the room just above the Catacombs, located exactly one screen down, and one screen right from the Yellow Castle. Note: The pixel will be invisible unless you rub along a southern border.
7- Drop the pixel in this room with a barrier on the right side.
8- Retrieve two more objects (Key, Sword, Chalice) and drop them in the room.
9- Once there are three objects in the room including the pixel, you can walk through the rightmost wall to view the Easter egg.

(Steps 1 to 9 are from a guide written by IGN)

The developer room in Final Fantasy II/IV on the SNES was always a favorite and I loved that the remakes/re-releases would update the room to include different staff that worked on the newer versions.

Finding the Kilrathi ship (Wing Commander series) in Ultima VII. When you find it, the game even plays the Wing Commander theme

Quake -- dopefish!

The one that always stood out the most to me was in Shadows of the Empire- a game with fantastic flying controls, but TERRIBLE on-foot controls! There was a cheat in the game that allowed you to skip all the vehicle levels (the highlight of the game!) and only do the on foot stuff. For flying fans it was devastating, BUT: at the end of the game, all is forgiven thanks to an amazing Easter Egg:

The final level lets you take control -at last- of the freighter that's been carrying you around the whole game, and you're supposed to use it to fly into the core of a space station and destroy it while surrounded by a whirling dogfight of X-Wings and TIE Fighters, Return of the Jedi style. Activate the right button combo though, and the camera button becomes a switch that magically transforms your lumbering old freighter... INTO an X-Wing. Hold it again, and it becomes a TIE Fighter! Best of all, you won't get penalised for who you shoot down in the dogfight, so you can pick a Rebel or Imperial fighter and go on a killing spree of it's enemies... forever!

Now THAT... is how you reward your players who stuck it out to the end!

I would have to say my favorite Easter egg would be the hidden room in batman arkham asylum. Not only is there no sign that you can break the wall but you have to have beaten the game before you can find it. Just great way to hide a sequel plan.

My most memorable secret in a video game was learning that you could run into Jason in Dead Island. That was pretty awesome. The Mario paintings in Zelda: Ocarina Of Time were pretty sweet too.

I'm a big fan of stealth action games and accordingly my favourite Easter egg from a video game is Sam fisher's dialogue in one of the missions of Ghost recon Wildlands. During his missions ending debrief he mentions Snake from metal gear solid and him being last of the "army infiltration" operatives who active nowadays.

He's then informed that snake has been retired for a while now(nod to konami + kojima falling out?) which causes him to come to a sad realisation; aptly conveyed by him as "then it's only me...."

As a stealth games fan can be also taken as a meta commentary that games like old splinter cell and metal gear solid have all but gone now and i can't help but feel the same sadness as his character. :(

I don't know if it counts as a secret/easter egg, but I loved how Ciri alluded to Cyberpunk 2077 in The Witcher 3, when talking about her dimensional travels. :D

Call of Duty Black Ops isn't on GOG, but it's the only Call of Duty game I know where the main menu has Alex Mason (the protagonist) tied up in a torture chamber, and you navigate the menu options on a TV screen. You can also look around the room through Mason's perspective. By looking down, you can see his hands all tied up. That's the only hint that you can have Mason wrestle out of the bonds tying his hand, but it's something you can do and really simple. Mash the buttons that make his left and right hand struggle (R2 and L2 on PS3). Broken free? Good.

There's not much to do in the torture room, again, the game doesn't give you a single clue about what you can do here...well, unless you wander around and discover there's a computer all the way in the back. And you can use it! The computer is of course running DOS and has data from before and after the game's main events, as shown in the emails and files when you log in as Mason. All this makes the game a lot more fleshed out, and there's even an entire "hacking" minigame where you try and figure out the passwords to other accounts on the computer through exploiting human laziness in coming up with and remembering good passwords.

It gets better, though. The computer has Zork I installed, the whole game, so you can scream into the void and loot abandoned houses - complete with the "save" and "restore" commands. This is the only way to play Zork I on consoles, by the way. Also included is a 3D minigame which is basically CoD's zombie mode but in an arcadey, top-down, twinstick shooter format.

I swear I spent several hours doing what I can on that computer instead of shooting bad guys - you know, the thing Call of Duty is known for.

(If you're playing on console, HOOK UP A KEYBOARD. You can even press up and down to quickly access recent commands, but using a full-blown QWERTY keyboard with a controller is painful.)

My favourite is the weird Jesus painting in Hitman 2