GTA San Andreas - "There are no Easter Eggs up here" Easter egg:
When you climp up to a bridge in San Fierro, you're a greated by a message "There are no Easter Eggs up here. Go away". The developers created this funny easter egg knowing the players will look for one.
Assassin's Creed franchise:
Did you know that every character you kill in any Assassin's Creed game was a real historical figure, even the location and the death of that character are the same as the ones in the game! Fascinating!
Fallout 3 - "The rude Easter Egg":
If you ever find a hidden door and open it, you'll come across a not-so-nice suprise. You will be greeted by a message saying you to have an intercours with yourself (fu*k you). Not so nice developers!
Half Life 2 - "The backwards Easter Egg":
If you ever come across those Headcrab zombies, and you will, you'll definately hear that they are saying someting inaudible. However, all they are screaming is "God help. Help me!" backwards!
Crysis 2 - "The rave elevator Easter Egg":
While playing the mission "Dead man walking" in the office building, you'll come across a lovely suprise: 2 soldiers dancing in a small elevator. All followed by disco ball, rave music and fog!
Borderlands 2 - "The Minecraft cave Easter Egg":
While playing the "Caustic Caverns" mission you'll find some dirt block in your way. Break them and you'll enter a cave designed in pixelated "Minecraft style" filled with creepers. Also, killing the creepers will give you a pixelated Helmet for free!
Halo 3 - "The cavemen familly Easter Egg":
In Halo 3's mission called Sierra 117 you'll find a thought-provoking easter egg - a family of cavemen monkey-look-a-likes (mum, dad and their 2 children). It's not known why developers placed this easter egg here, but it sure is funny!