In Aladdin, on the first desert level, look at the columns below the arew with the store. There is a picture of Goofy on one of them.

In World of Warcraft there's a string of quests that is full of Easter Eggs from the original Legend of Zelda. You fight a boss named for the first boss of the game, search for a boomerang, the character's name is Linkin, etc.

I kind of geeked out when I realized that.

The same area of the game has characters meant to be Mario & Luigi as well.

Oh my, I found a borked Easter egg ...
http://img174.imageshack.us/img174/8291/eastergq4.png
:(

In Worlds of Ultima: Martian Dreams: there's a spot in the plains where you will find a fairly large "x" on the ground. If you dig at the center, you will find a pair of ruby slippers. Put those on, and you will be informed that you have won the game. Then you will be treated to a short slideshow of artistically rendered shots of Kansas. :D

In Ultima 6 you can murder Lord British by causing a plaque in his castle to fall on his head. After that you'll hear the Guardian say "Yes Avatar, that's the right thing to do."

This is alluding to an incident in the ORIGIN offices where a plaque in the building fell on Richard Garriott's head and left him with minor injuries.

Lawl.

Isn't that one in Ultima 7?

I found it fun that you can kill Lord British in all the games, especially when I first killed him in Ultima 7 by accident...

In Chex Quest, in level 2(the storage facility) you go into one of the rooms with the elevator at one side, and there's a stack of crates right next to it, if you go onto the elevator and run off with the right timing, you can get on those crates, there is then a secret door that leads to a room with portraits of the Digital Cafe staff, and there's the LAZ device(bfg9000)!

Also, in The Legend of Zelda: Link's Awakening, you can steal items from the store, but then everyone calls you by THIEF.

Is there a Dopefish? or a Dopedish?

In gamefaqs.com there is a full guide walkthrough written by yours truly for the game and the expansion with all the easter eggs I was able to find. I'd like to add that one as I didn't know that :(

(Shameless self promotion)

More like a secret then an easter egg, but in Doom II one of the secret levels is a replica of the first and last levels of Wolfenstien 3d from the first episode, complete with soldiers from the game. Grosse was replaced by a cyber demon. At then end you got to kill 4 Commander Keens that were hung.

Two other ones were in Doom 3. Type a cheat from Doom I or II in the console and it will say "You remembered well". Another is near the end of the game is a secret passage that leads to a ritual altar with a PDA on it that has messages from the developers.