Easter Eggs

Dual Eye Patch

You give Ulgrim the Unpleasant Eye Patch.
Ulgrim the Unpleasant tells you, "Yar, now I be needin the patch for the other eye. Har-yar and Dar!"
You give Ulgrim the Unpleasant Eye Patch.
Ulgrim the Unpleasant tells you, "Yar, I be making your special Eye Patch now. Here ye be."
Ulgrim the Unpleasant gives you Dual Eye Patch.

Colosseum

There are three secret bosses in the Colosseum that can be accessed with special items. The statues for them lie behind the door at the end of the east wing. Once you have given one of the three items you can use the door. More details here.

Drunken Madman

You give Drunken Madman Apple.
Drunken Madman says, "Entering damn portal-space!"
Drunken Madman crosses his arms and raises into the air as he enters portal space for a few seconds

TARDIS

A police box appears extremely rarely in game. The only place with pictures of this is Maggie's.

Tuskers

Orion revealed an easter egg in a blog about the 100th patch. - Link

You give Brighteyes, the Tailor Mushed Nanners.

Brighteyes, the Tailor says, "It's people! Mushed nanners is people!"

Brighteyes, the Tailor says, "It gets very lonely out here. I've had a lot of time to myself."

Violet Ball Interactions

Surloshen Realaidain:
You allow Surloshen Realaidain to examine your Violet Ball.
Surloshen Realaidain tells you, "So lovely! This would look great next to my lilies! But I cannot take that which is not mine. Good day to you, my friend."

Crystal of Life Protections in Chapterhouses:
You give Crystal of Life Protections Violet Ball.
As you press the violet ball against the crystal, it is slowly absorbed. You perceive no other effect.

From Missing Person Investigation:
You allow Guard Hrothgar to examine your Violet Ball.
Guard Hrothgar tells you, "Aha! A clue!"
Guard Hrothgar tells you, "Wait...this isn't a clue."

From Tanada House of Pancakes Quest:
You allow Chow Doun to examine your Violet Ball.
Oh, come now - do you think we'd still keep adding violet ball interactions after we told the community that we were doing it? Err....apparently, we would.

You give Corpse of Pon Mi Violet Ball.
You plant the violet ball on the corpse. In a thousand years, archaeologists will stumble upon the corpse and think that the violet ball was the lynchpin to some horrible human sacrifice ritual.

You give Pon Mi Violet Ball.
Pon Mi looks at the orb, then at you.
Pon Mi tells you, "What am I supposed to do with this? Throw it at the Tanada who come to kill me?"

Camping Ineptitude

See: Camping Ineptitude Spell

Several chests across Dereth have a unique spell they cast on whoever opens it. The face in the spell effect is Allen Grey, the former site manager of the old Crossroads of Dereth circa 2000.

File:Camping ineptitude effect.JPG

Pet Rock

On Dark Isle is a small rock at 86.9N 59.8E. If you use it you get the following:

You pat the top of the stone. Good stone. I mean, what else could you expect from a stone? You wish you could take it home and make it your pet.

Credits

If you wait all the way through the credits (Click Credits button on main character login screen) after a while the following Easter Egg appears:

Wow You sat through all the credits ALL of them. U R t3h h4rdc0r3 Seriously though. Y'all rock. All of you. Whether you're playing AC for the first time or you've been with us since beta, we're thrilled that you're here. All the players, everyone who's made the community so active and fantastic, everyone who's given a part of themselves to this game. We couldn't have done it without you. Thank you. (The credits are over now.) (Seriously, go kill some Olthoi!) (Fine, you can stick around, but I'm gonna go hunt Eaters.) (Ibn out.)

Other

The Golem Sanctuary dungeon has a hidden message in its map.