Team Fortress 2 for those who pre-ordered Left 4 Dead 2 from Steam or "participating retailers." Though the hat does ... apparently nothing, it implies – in the immortal words of Destiny's Child – "I'm not gonna give up, I'm not gon' stop, I'm gonna work harder, I'm a survivor."
Some may hold issue with the fact that the Team Fortress and Left 4 Dead universes have collided; however, Valve has explained the phenomena, stating that "while conducting a science experiment on parallel dimensions, Bill's hat fell off a table into the Team Fortress universe." Done! That's all the justification required. Oh, and if Bill's hat isn't enough incentive to pre-order the zombie apocalypse survival simulator, know that the game is currently 10% off on Steam.