The End Can't Come Soon Enough
Game: Metal Gear Solid: Snake Eater 3
While this isn't a true "Easter Egg" per se, we had to include this as one of those instances where Kojima's insanity tilted in the right direction. The Boss Battle with The End is pretty epic, but there's one simple way to take him out (OK, there are actually two; you can snipe him earlier in the game, you cheapass).
Once you begin the fight with The End, turn off the game and either wait a week or fast forward your PS2's clock to one week in advance. Turn the game back on, and The End will have died from old age.