My favorite easter egg is the Mario character paintings in the window of Hyrule Castle Courtyard in The Legend of Zelda Ocarina of Time.

Leisure Suit Larry 6:
The shower stall has a tile peep hole to peep at the ladies bathing area. Was pretty funny. One of many naughty moments in gaming easter eggs.

Perhaps lesser known is another favorite game from Naughty Dog: The Rings of Power for seg genesis. If you press a button combo on the 2nd controller, the game will show the female character in the opening sequence topless! Gadzooks!

My favorite easter eggs would be the entirety of the hacking system in Enter the Matrix, especially the messages from Spark and Morpheus and using the sword in the game.

My favorite Easter Egg was in X-Men Origins Wolverine.
A reference to a weapon and character from World of Warcraft.

Frostmourne sword hidden in a cave and the skeleton next to it, that is meant to be Prince Arthas.

I know Wolverine doesn't need a sword but a really, really wanted to use it!

that sword on elden ring, that got redesigned to look like gut's sword. also, like its not official on that very same game there is another sword that looks like a little homage to blasphemous (game kitchen's game)

For me, it would have to be one of the first known easter eggs in the Atari game Adventure, being that of the name of the programmer, Warren Robinett. This was a game I spent many hours playing as a child and was one of my absolute favourites on the console. In fact, I occasionally fire up the Atari emulator, Stella, and play through it for nostalgia's sake.

My favourite secret would be a secret true ending of “Prince of Persia: Warrior Within”. In order to get this ending one has to find all nine health upgrade pedestals and the Water Sword. Then the player will be rewarded not only with a different ending, but also with a different and arguably more difficult foe to fight during the final battle. I love this secret because of how impactful and rewarding it is.

Most secrets in other games merely feature a variety of consumables (exempli gratia, almost all secrets in classic games like Doom, Quake, Serious Sam, etc. offer health, armour, ammunition, and so on), upgrades (like upgrades for the blaster in Unreal Gold) or other bonuses, that make games easier, thus actually taking away some of the enjoyment from those games and as such they do not really reward curiosity and determination required to find them. It may be fun looking for them, but when you find them you realize that the game became much easier. This always makes me wonder if it is a good idea to look for such secrets in the first place.

Other secrets may reference objects and subjects from the outside of the game world (for example, a whale in the desert or a police box from doctor who in the original Fallout games). Even though they are quite entertaining, especially in those games that are not very serious, nonetheless they are arguably immersion breaking. Moreover, none of the aforementioned secrets offer an additional challenge and a true ending as a reward.

In “Warrior Within” one has to find those pedestals, figure out solutions for the puzzles and perform difficult acrobatics to get the upgrades. At first life upgrades may make game a bit easier, just because the prince will have more health, but in the end one’s determination to finding all the upgrades is rewarded with an ultimate weapon – the Water Sword, and what is even more important, with a different challenging final fight, as well as with a true ending. Just look at all the other games. How often will you see a game developer, who creates a canonical ending only and purely for the dedicated appreciators of their work?

To summarize the above, I love a secret true ending of “Prince of Persia: Warrior Within”, because it manifests a great effect on both, gameplay and narrative of the game, as well as rewards a strong desire of a player to explore the game and to face and overcome all the challenges the game has to offer. Finally, it shows that the developers had a great determination to deliver a challenging and rewarding product and a strong appreciation of those who will play the results of their labours, as they decided to design that final fight and write and implement that ending, which only few will ever experience.

One of my favorite easter eggs occurs in Terraria. The game is alive with critters, such as bunnies and squirrels, but the ducks are the loudest. They quack and they quack alot! When I'm gaming, I almost always wear a headset, so it's crazy how much these ducks quack. But, once in a while, you'd hear the voice of a man say the word QUACK, in a coarse tone. The first time I heard it, it made me laugh so much, as it was unexpected and gave the impression that one of those ducks was an imposter. xD It was such a simple easter egg, but very memorable.

I later researched it and learned that there was a 1/300 chance the sound would play and that the Re-Logic team pulled this sound from a royalty-free site because they thought is was funny.

I normally appreciate games making references to movies since they were a important part of my upbringing, Horror in the 80s was a big part of that. Playing Lovecraft's Untold Stories (came out in 2019) is an eldritch horror action roguelike that is procedurally generated.

In the second level “The Hospital” as the Detective character you can encounter a room with a single scientist body laying on the floor, when interacting with it, your character remarks that if you could reanimate it maybe it would be a less violent zombie and could get some information.

Going through the level itself if you scour every corner looking at all the shelves and cabinets you can find an empty syringe with an odd design. If you are through with your search, you will find another room with a machine filled with a green glowing fluid labeled reagent combining the syringe with the machine will give you the reanimating serum needed to revive the corpse.

Returning to the corpse and injecting it will revive the scientist and be welcomed with the familiar looking face of Jeffrey Combs who played the character of Herbert West (a medical student who invented a serum which can re-animate fresh corpses) from the 1985 movie Re-Animator which in itself based on the short story from H. P. Lovecraft from 1922. After gathering some lore about a secret laboratory and a passcode, the scientist turns into a mindless zombie that you have to put down and collect a passkey. This opens a complete other level where you delve deeper into the basement of the Hospital where you encounter a different boss.

I do love this easter egg/homage since if you are the type of person that likes to search every nook and cranny, it rewards you with this awesome interaction from a great movie. The game generates random maps every time you play it so finding the items to get this easter egg is kinda of a pain but well worth it to me since initially it brought a crooked smirk to my face making me have nostalgia.

I remember one from golden era of Win95. Not sure about name right now. Although it was an desktop application, not an game, I think it's worth to be mentioned.

In one of pop up window was tiny π symbol. When you pressed Ctrl+Shift and clicked that symbol, the real magic started :)

If you're unsure what π means here is hint: https://imdb.com/title/tt0113957/

GOG reference in a Witcher3 journal entry:

"The Defensive Regulatory Magicon (or DRM for short) belongs to the above mentioned group of the longest-lasting, most effective and hardest to break defensive mechanisms. In order to recognize the individual administering it, it makes use of a portal mounted at the entrance of the area it is to defend. This portal passes streams of magical energy through the body of the person entering and can, in the blink of an eye, determine if this person has the corporeal signature (eyeball structure included) of the entitled administrator. As a result, the only unauthorized individuals that can possibly hope to enter are mimics.
DRM thus makes for an extremely effective and near-unbreakable security measure – but you are in luck, for you hold in your hands the key to bypassing it, namely the present tome, Gottfried's Omni-opening Magicon, or GOG for short. In the pages to follow you will find innumerable methods for deactivating DRM, or, even better, bypassing it altogether"

The coolest experience I've had with hidden content in a video game was being introduced to Thera in EVE Online.

Like any space game, EVE has a galaxy of star systems with known connections between them. However, EVE also has wormhole space: over 2,000 star systems that are in constant flux, forging and breaking random connections with regular systems and other wormhole systems. To get into and travel through wormhole space requires a complicated scanning process, so your ship has to be built for the task, and after finding a wormhole that will lead to another system, you have no in-game way to find out where it will go. Spend a bit of time in and travel through wormhole space, and your odds of getting out the way you came in start dropping dramatically. When this happens, finding a way out is a puzzle, and it's entirely possible to get stranded in wormhole space if you run out of supplies. With no law enforcement, your odds of survival also start dropping dramatically. Travelling through multiple wormhole systems in search of a wormhole back to realspace is a good way to get discovered by a cloaked ship lying in wait for easy prey.

All this being said, the majority of players are unaware of wormhole space, avoid it, or spend as little time in it as possible. Jumping into wormhole space and looting ancient ruins was my chosen profession, but even I would dip my toes in as little as possible before running back to the safety of realspace. However, there's a separate breed of player -- one who calls Thera home.

To a casual player, "Thera" doesn't mean much, but once I understood that it was a star system being referred to, it began to hold a mystical, legendary quality much like "Shangri-La": a place only heard about in the myths and tall tales of grizzled explorers and long-time space captains, which doesn't appear on any map. In my excursions into wormhole space, I had never happened across it. I had traveled through thousands of systems without encountering it, and as I learned more about it, I kept an eye out for it, but to no avail. Then, one day, I was being mentored by a more experienced explorer player, and he told me exactly what it was.

The 2000+ systems of wormhole space have no regular stations. Player-built stations are allowed, but are almost always private, can be destroyed in PvP warfare, and unless you're in the guild that controls them, really aren't a safe place to keep your stuff. However, there's a single neutral location in wormhole space, a single system which has the regular NPC-run stations: Thera. Discovering it without seeking it out is unlikely, and casual explorers like myself staying in the low-difficulty wormhole systems will never happen across it. But the regular denizens of wormhole space have a chance of finding it, and with enough players . . .

EVE Online has a player-run coalition of wormhole explorers, the Signal Cartel. They rescue players stranded in the chaos of wormhole space (finding these players can take days), maintain supply caches in about 1/8 of wormhole systems to assist those players (because finding players can take days), and, most crucially, track the wormholes they travel through, and which systems they connect to and from (so that finding players only takes mere days). It's this navigational information which allows Thera to be navigated to by scrubs like myself, and makes wormhole space a viable parallel galaxy to the regular one.

My experienced explorer friend guided me through the process, and after a dozen jumps and a few finicky scans, there it was. A haven in the midst of chaos. After docking, I discovered the people there were some of the chattiest I've ever seen, likely due to the isolation. At the same time, the system was a Wild West -- the people there would just as soon chat with you as kill you. Getting in was sketchy but doable. Getting out was potentially lethal, as PvPers liked to sit outside the stations and take potshots as undocking ships got up to speed.

As I sat there in the station for the first time, my explorer side basking in the fulfillment of myth and destiny, my other side, the freight-trading side, began looking through the market and thinking madly. Supplies in Thera were low, and prices high, but not for lack of sales. This was the greatest arbitrage opportunity I had ever seen. A month later, I had taken to blockade-running ships into Thera to sell them for profit, and the system started to become mundane. But the myth itself wasn't mundane. I had grouped up with some friends who also played the game, and making mention of Thera, I could sense they felt that same allure of mystery.

My favorite secret/easter egg has to be the Serault War Table missions in Dragon Age Inquisition. They were only available if you played the Last Court text game through the Dragon Age Keep, which they have now discontinued. I had found the game completely by accident and played through it, then was pleasantly surprised when I saw that I had unlocked the missions in Inquisition. Another fun easter egg in Inquisition is probably the Krogan head in the Winter Palace. Gotta love when Bioware gives references to their other games.

One of my favorite secret/Easter eggs is the secret cow level from Diablo. It's just so silly, I can't help but like it lol.

I also like when I find characters from another game in the game I'm playing in general, that's always fun.

In Cyberpunk 2077, in the wall of a large residential building in The Glen, there is a graffity that most of you would know from memes, it says: "EMOSIDO ENGAÑADO", which is (broken) spanish for "we have been deceived". Suffice to say I walked in front of the original real life graffity everyday on my way home (it was made by a group of families who were promised a social housing in an illegally occupied group of apartements, and felt betrayed by the town hall and the banks). I never imagined that a part of my town could end up in one of my favorite games in years, so this eastern egg is very close to my heart, and find it very fitting in a game about corporate greed and criminal underworlds of the future.