What are some others that you might know?
Many of the earlier examples were the only way that programmers were able to get credit for their work. The first one (they say.. ) is from adventure on the 2600. which in itself was an adventure to find.
Here are some other retro game easter eggs
on the 2600

Adventure:
Inside the black castle catacombs (on difficulty level 2 or 3), embedded in the south wall of a sealed chamber (accessible only with the bridge), is an "invisible" 1-pixel object referred to as the grey dot.[6] One must "bounce" the player cursor along the bottom wall to "grab" the dot. The dot is not actually invisible, but simply the same color as the wall and is easily seen when placed in a catacombs passage or over a normal wall. The dot is not attracted to the magnet, unlike most other objects in Adventure.

Bringing this dot to the east end of the corridor below the golden castle while other differently-colored objects are present causes the wall object to also become 'invisible', allowing the player to pass into a room displaying the words "Created by Warren Robinett".

Carnival
Holding down the fire button as you power the system on causes a dot to appear in the top row of targets. Shooting the dot before it leaves the screen (it then turns into a duck) will reveal the programmer's name.

Yar's Revenge
Kill the Swirl in mid-air. Position Yar so that spot on it's back is in the same vertical plane as the black "streak". Programmer's initials will appear after explosion (HSWWSH = Howard Scott Warshaw)

Missile Command
SELECT game 13(?). Let missiles rain on all your cities and waste all your missiles without scoring any points. The rubble of the rightmost city will change to "RF", the programmer's initials (Rob Fulop)

Desert Falcon
Get a superpower and swim near the top of a lake (before you lose it) to make the programmer's initials appear. Easiest way is to get invincibility (bird-cane-feather).

Communist Mutants from Space
Hold down the joystick button while loading and the "HI" on the score screen will change to "SHL", the author's initials. (Steven H. Landrum)

NES

http://i.imgur.com/Bnx2T.jpg

I know a TON of easter eggs to a lot of the zelda games, does that count? I'll just list a few off of Ocarina of Time, though...

1. The most memorable one for me, the fact that older Malon's necklace is a gold Bowser head.

2. If you can locate one of the many "seams" inbetween the game's polygons, and if your careful, you can walk up it and reach places the game doesnt want you to go! (I always like walking up the hills as a kid outside Hyrule Castle and looking at the back half of the castle, which btw is completely empty! The polygons back there don't even exist, cuz you cant see it from the front anyway!)

3. No matter how fast you are, no matter how many times you try, when you race that guy from the gerudo valley to kokiri forest, he'll always win (the programmers never figured out a prize for this, so they purposely made it impossible) Oh and if you try warping, it just takes up more time than normal (since the clock just calculates how long it would normally take to run to that spot)

The one that I remember the most would have to be the easter egg in Kirby's adventure for the NES.
If you grabbed the Cutter power, killed a certain enemy, a warp star that you had to take would disappear, therefore letting you keep on travelling to the left of the warp star. once you reach the end of the left path, it says HAL in kirby bricks, an easter egg that the HAL producers put in the game.

Originally posted by Geldra

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The one that I remember the most would have to be the easter egg in Kirby's adventure for the NES.
If you grabbed the Cutter power, killed a certain enemy, a warp star that you had to take would disappear, therefore letting you keep on travelling to the left of the warp star. once you reach the end of the left path, it says HAL in kirby bricks, an easter egg that the HAL producers put in the game.

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That was an easter egg? I found that when I was like 6 years old and didn't even know what I was doing! I didn't even know there were such things as secrets like that in games back then too. Lol. Cool!!!