It's become something of a tradition ‘round the Tower to observe Easter with, what else, lists of Easter eggs. So here is my dirty half-dozen, from well before the 8-bit days.
When I went on my hunt for this list, I needed a theme, so I settled on "Easter Eggs I have actually observed." Having missed most of gaming in the 1990s thanks to work, college, and not owning a PC - and because you know about all the cool recent Easter eggs - we'll have to reach way back to antiquity.
Of course, commenters are invited to share with us the oldest Easter egg you can recall finding. Or just any old cool Easter egg you remember, video game or otherwise. In fact, the coolest Easter egg I can recall is the one my brother and I painted to look like Jesse Helms back in 1984. Had glasses and everything. Then Fletch smashed him against a tree.
Where was I? Oh yeah, old Easter eggs.
Smurfette Topless (Smurf: Rescue in Gargamel's Castle, Colecovision)
Yes, Smurfette porn long, long predates the discovery of Rule 34. This was a redraw glitch available on the final screen of Smurf: Rescue in Gargamel's Castle (in which Gargamel makes no appearance.) If you walked off the screen to the preceding one, then back very quickly, the slow redraw would leave Smurfette topless. The topless Smurfette Easter egg - with the false urban myth that she remained topless and would slap the Smurf out of your Smurf - was all the rage in fifth grade. I wouldn't discover it until I bought a ColecoVision off of a seller on Usenet (yes, really) my senior year of college.
RF (Missile Command, Atari 2600) Warren Robinette's secret room in Adventure is probably the most notorious Atari 2600 Easter egg. This one is a close second. I forget where I first heard how to do this, but it increased my neighborhood cred a thousandfold when it went off, as described, the first time. Select difficulty 13, and fire off all of your missiles without scoring any points. At the end of the game, the initials of the programmer, RF, pop up from the ruins of the rightmost city. The coder's name? Rob Fulop, who also wrote "Demon Attack" for Imagic, and was Billboard Magazine's 1983 Programmer of the Year.
"FEEL DESTRUCTIVE "(Pirate Cove, VIC-20) Pirate Cove was a cartridge text adventure for the Commodore VIC-20, and one of the first pieces of software Dad bought me and my brother for our first ever computer. Pirate Cove's command line was brutally unsophisticated. Two words at most, verb and object (sentence diagrammers, the subject was always implied to be "you.") For a nine year old trying to figure out how to get past the goddamn crocodiles, without any visual assistance, it was hard to imagine any solution other than "KILL CROC." But commanding the game to KILL anything brought up this peace-love-dope reply: "I'm sorry, I can't do that. I don't feel destructive." You know where this is headed; I typed FEEL DESTRUCTIVE. Came the reply: "OK, POOF! The game is destroyed!" And Dad nearly herniated himself laughing at me.
HSWWSH (Yars' Revenge, Atari 2600) Another less-publicized set of initials. These are for programmer Howard Scott Warshaw. You had to wait for the Qotile to go into swirl mode, then hit it in mid-flight with the cannon. During the explosion animation, a black line would appear in the midst of the radioactive cloud. Hovering over it brought up the initials and ended the game.
Ghostbusters Scam (Ghostbusters, Commodore 64)In Activision's original Ghostbusters game, you earned money from mundane ghost-busting missions to buy equipment and vehicle upgrades. You started the game with a pitiful sum, barely enough to buy the Ecto-mobile. But in the new game screen, where you create a "bank account" to start the game, punching in "BELLIN ADAM" - for programmer Adam Bellin - and the number 12345 got $954,000 dumped in your account. Then you could go out and buy the Porsche, the mobile ghost detention unit, and every other high dollar gadget you needed.
Indiana Jones and the Extra-Terrestrial (E.T., Atari 2600) Howard Scott Warshaw was so proud of his role in the cartridge that would destroy Atari (the first time) that he upped the self-referential Easter eggs to two: the pieces of the phone were his initials, and there was another way to get the letters up on the screen. You could also summon other Steven Spielberg IP. After collecting all the phone pieces and giving Elliot seven pieces of candy, reviving the flower turned it into a Yar. Doing it a second time changed the flower into Indiana Jones, as represented in the 2600 Raiders of the Lost Ark game. Doing it a third time brought up Warshaw's initials beside the score, so you knew whom to credit blame for this masterpiece.