Borderlands 2 has a lot of hidden secrets, secret messages, references to popular culture, and other types of Easter Eggs.

This is the list of Easter Eggs in Borderlands 2

References to other games

Skyrim

Bullet to the Knee - A Sanctuary Citizen tells you "I used to be a Vault Hunter like you, until I took a bullet to the knee". Reference to Skyrim's famous line that the guards say, "I used to be an adventurer like you until I took an Arrow to the Knee"

Screenshot of the Sanctuary citizen who says the meme

Donkey Kong

Donkey Mong - A Bullymong named after the enemy from Donkey Kong, it even throws barrels at you. There is also the achievement you get for killing it, "Definitely An Italian Plumber", which is a reference to Mario.

Duke Nukem

derp Nukem - A Rocket Launcher with the Duke Nukem logo on it, causes a nuclear blast visual effect.

Frogger

Gaige's Frogger Head - Gaige the Mechromancer has a head called "Fragger" which looks like a frog

Minecraft

Blockhead - Shotgun, Fires bullets in a Block (3x3) pattern. Also has red text: "Also try Minecraft!"

References to movies and comics

Batman

Rakkman - A reference to Batman. Killed his parents, lives in a The Rakk Cave, and comes complete with a Rakkarang, Smoke Bomb, and a colony of Rakks.

3:10 to Yuma

3:10 to Kaboom - This mission is a reference to the famous movie, 3:10 to Yuma. Both deal with a train.

King Kong

King Mong - A reference to the classic "King Kong" movie.

Dirty Harry

The Unkempt Harold pistol says: "Did I fire six shots, or only five? Three? Seven. Whatever.", A reference to the Dirty Harry Quote "Did he fire six shots or only five?"

Logan's Run

The Dynamic Logan's Gun pistol says: "Gun, Gunner!", A reference to the Logan's Run Quote: "Run, Runner!"

Se7en

There is a pistol in a box called Gwen's Head

QR Codes

In Dr. Zed's Clinic QR Code - It says: "Thank you so much for playing our game, now I'm rich with love AND money! !!Nick Wilson!! Ditto!!". Nick Wilson is a Geabox Software Developer.

Other References

Heyo Steve - You find Steve in the The Holy Spirits. "Heyoo Heyooooo Heyoo Heyooooo"

Impaler: Red text says "Vlad would be proud.", a reference to Vlad the Impaler

Whiskey Tango Foxtrot: The "WTF Shield"

Fabled Tortoise: Red text says "Win by a hare", a reference to the famous fable, The Tortoise and the Hare.

Wastelander Panda - Reference to Arcayus the Panda