One easter egg I found, (not taking credit incase somebody already posted it) but when I was just jumping around on the first earth level at the beginning, there is a light above a door that you can jump on. Get on top of the building to your left with the jump then crouch, then jump again for good measure. Look to you left for a short hallway like area. At the end, there is a skull, grenads, and a camera thing. Pick up the skull and the entire screen flases and it says blind. No you have nothing on your screen. No shield indicator, ammo, life, radar, or grenade indicator as well. I don't know if this is all it does or if you finish the game will it do something. But it was fun for a while. It really helps with the aiming though. You have to try and guess where to shoot. Like it would be in real life.
Thanks.