The Crysis 3 Nano-Decryption Easter Egg game. It's the same premise as the D.R.P mail unlock, but repeatable with varying levels of difficulty.
As with the credits asteroid game, which I posted earlier, I'm surprised this one has gone unnoticed. It's quite fun in a keyboard snapping sort of way.
Unlocking is easy. Go to the 'Extras' menu and sequentially press; up, up, down, down, left, right, left, right, B, A.
...if anyone completes the impossible mode with a keyboard, post a video!
