Easter Eggs you've found in games.

[Book Smarts]

Diablo III has a pony ref... but you'll have to find it on your own, noting that it can't be posted on the forums 

[alexandas]

in a certain level in portal prelude (might be portal probably not) theres a flash of white light when you go down a hall and it shows a mario like place with three warp pipes to go to different test rooms

[CyanideInsanity]

Well, with the recent player owned ports update to runescape, there has been quite a number of pony references.
Several islands names are references to the mane 6 -

Tuai Leit:the center of study in the arc region
Ai Jei: and island that grows apples
Rei ti: has high quality gems

These are the only islands in the region my map has access to, but there are a few more islands,  Kei Pi and Flou Tar-Shei, though I can't view their description ingame, and the RS wiki isn't too helpful there.

And just because, here is a screenshot from the behind the scenes video of this update, featuring the developer of the ports.

Spoiler: Mod Moltare is win • show

Also this has started quite a lot of controversy on the RS forums, though I do find it a bit funny, after all every update usually gets treated as runescape's own apocalypse.

[Blues-Music]

Diablo III has a pony ref... but you'll have to find it on your own, noting that it can't be posted on the forums 

Oh yeah the secret pony world....

[Tara]

In Super Mario Galaxy 2, in one of the levels, if you switch to first person view and look in the distance on the cliffs, you may find these weird shadow people.

They look like aliens, to me.

What makes them even more creepy is that no matter where you are in that level, you can see them off in the distance watching you. 

Not sure if it counts as an Easter egg, but if you type 'Pewdiepie' in Scribblenauts Unlimited, you'll spawn the Youtube Let's Player. I tried to find a screenshot with Google, but couldn't and my gaming computer is being repaired right now. The character looks... interesting to say the least. xD

In Super Smash Brothers Brawl, you can see a photo of a developer's cat hiding in the shack in the pokemon stadium level.

You know where you find Princess Zelda in Ocarina of Time? When I was a kid, I looked through one of the windows  on the caste walls in the area where you first see her as a kid, and I saw pictures of Nintendo characters aligned on a wall inside. I think that was the very first Easter egg I've ever found, and I remember boasting about it to everyone I knew. xD

I don't know if I figured this out or not, but there's another window at the other side of the area with the princess. When you look inside, it's just an empty hallway. But if you shoot it with a slingshot, a guard will poke his head out and throw a bomb at you. How rude! 

[Jenzy]

In Paper Mario: Sticker Star, I didn't even know it used the Gyro sensor.

If the HP Bar is showing, shaking the system makes it swing (I thought it only moved when you got an "Excellent" on the attacks to show that it shook the stage a bit from the power). And if you have Shiny/Flashy stickers on the bottom screen, tilting the system up and down makes a reflection effect as if you were looking at real stickers (You know. The Glare when you look at the sticker from different angles)

[Nala Valor]

When you had a gameshark for the game LoZ Ocarina of Time, you could trigger an event about a game that was going to be announced a few months after OoT. The thing you spawned was a space ship from Star Fox 64. You had to defeat it and it gave you a 200 rupee when you defeated it. It had a lot of HP but did only half a heart.

[Book Smarts]

The original Paper Mario had the Koopa Brothers, they were most likely a cameo of the Mutant Ninja Turtles.

[Little Judas]

The Elder Scrolls IV: Shivering Isles has a chest in Xirethard called the Dark Chest of Wonders.
Which is a song from Metalband Nightwish.

One could be sceptic and say it is coincidence but it contains the Ring of Oceanborn.
Which is an album from Metalband Nightwish.

[CyanideInsanity]

When you had a gameshark for the game LoZ Ocarina of Time, you could trigger an event about a game that was going to be announced a few months after OoT. The thing you spawned was a space ship from Star Fox 64. You had to defeat it and it gave you a 200 rupee when you defeated it. It had a lot of HP but did only half a heart.

Actually star fox 64 was released before OoT, and the enemy movements from SF64 where used for Volvagia's flying cycle. So all they did was swap out the arwing model for Volvagia's. A lazy thought, but in the industry this kind of laziness helps.

[Detective Woona]

The Elder Scrolls IV: Shivering Isles has a chest in Xirethard called the Dark Chest of Wonders.
Which is a song from Metalband Nightwish.

One could be sceptic and say it is coincidence but it contains the Ring of Oceanborn.
Which is an album from Metalband Nightwish.

GREAT SCOTT!! I never made that connection! I just sat there and thought, "Now why would they name this chest?"
Nightwish references in Shivering Isles....sneaky Bethesda.

I'ma go listen to Dark Chest of Wonders now.

[Book Smarts]

If you beat Mario 64 and have all of the stars, you can use the cannon outside of the castle to reach the roof; where a yoshi is wandering. If you talk to the yoshi, he'll reward you with 100 lives

[Chishio Kunrin]

If you beat Mario 64 and have all of the stars, you can use the cannon outside of the castle to reach the roof; where a yoshi is wandering. If you talk to the yoshi, he'll reward you with 100 lives

I've seen a screenshot of that with a scumbag hat put on Yoshi. "Gives you 100 lives after you beat the game".
What's the point?

It's not like Okami, is it?
When you beat Okami, you get to replay the game on a special sparkly save slot that it makes, and you start out with all of your weapons except the final ultimate ones. Depending on the number of beads you found and stuff like that, you also unlock some things, including the ability to turn your character into different dogs, a stone wolf, a regular white wolf, or a somewhat realistic-looking wolf.

If you found all the beads, you get to play with infinite HP.

Also, after you beat Okami, you unlock another part of the main menu where you get to listen to all the music from the whole game. I think you also unlock getting to look at concept art.

[Nala Valor]

The Mario 64 thing was to reward those who knew how to make the roof jump. With no star, you could speak with yoshy.

[Nala Valor]

I just found out about an Easter egg that is in a good part of nintendo's game. The Tanoko's song. Tanoko works for Nintendo for the music and hid his song in a bunch of game.
Mario paint= press the O ((ps, this is possibly the first Easter egg made))
All animal crossing= find the dog guitarist who have Something close to Tanoko's first name. Request his song. He will ask you where you heard of that song and he'll play it.
Luigi mansion= at the place where you first see the control, wait 2 min. And a half and you'll hear the song
Link's awakening (XD too)= find a house with a bunch of frogs in it. Wait 2 min. You'll hear it.
Yoshy story (N64)= go on the training thing where you see a level selection thing. Wait 2 min and you'll hear it.

Almost all the time Tanoko participated in a game, he put that song in the game. There is rumors that it is on wii sports since he's in that too. There are a lot of other game like an other yoshy game on DS and one on the virtual boy that I can't remember how to get. If you have a Nintendo game and in the credits, there's Tanoko, check everything, everywhere!

((-EDIT-))
My bad! The name's Totaka!
And also, sorry I double posted.

[Jenzy]

I didn't find this. Someone else did (I don't own a Wii U)

Lazy/Clumsy Luigi thing with Nintendo (Near the end of vids)

[super_chris85]

finding Big the cat in Sonic adventure 2 for the dreamcast just randomly popping up in levels

[FeinSukanku]

I saw in a video that this guy's

necklace is the skull of another popular Sega character. It's blue, and has spikes down the back of it, so... yep. Sonic.

[Book Smarts]

There's a tauren marine in a campaign level of Starcraft II

[The Stars]

I saw in a video that this guy's

necklace is the skull of another popular Sega character. It's blue, and has spikes down the back of it, so... yep. Sonic.

... Consarnit Jetset Radio XD I wanna play that game so much now.

Anyways, may I quote Zer0 from BL2?
"This is the story
All about how my life got
Flipped, turned upsidedown."

... XD Fresh Prince of Bel Air