Team Fortress 2

Hats
 (or 
Headwear
) are items of equipment with their own 'head' section in the loadout screen. 
Hats have no direct effect on gameplay and are for cosmetic purposes only. They are unlocked at random during in-game playtime. 
Every 15,430 seconds (4 hours, 17 minutes, and 10 seconds) you have a 3.5714% chance of receiving a hat. Code wise, every 4 hours, 17 minutes and 10 seconds, the game rolls a virtual 84 sided die. If the number on that virtual die is a 1, 2 or 3, you will receive a hat.
In total, there are 30 unlockable hats as well as three unlockable 'hatless' models. Nine hats were first introduced with the Sniper vs Spy Update. Eighteen more hats and three hatless models were then released with The Classless Update. On September 2nd 2009, The Cheater's lament was awarded to all players who had not used an external idling program. Two new hats were also introduced during the 2009 Halloween event, the Ghastly gibus and the Mildly disturbing Halloween mask. In a similar fashion to the Gentle manne's service medal, the first possible method of unlocking the Ghastly gibus was to be one of the first 10,319 people to visit a hidden web page. The current method through which it can be unlocked is to complete the 
Ghastly Gibus Grab
 achievement bydominating a player wearing the hat. The Mildly disturbing Halloween mask can be unlocked by completing the 
Candy Coroner
scarechievement, which involves collecting 20 Halloween pumpkins from dead players during the annual Haunted Halloween Special event. Yet another all-class item, Bill's hat, was made available to all players who preordered Left 4 Dead 2 on Steam.

A Brief History of Hats

\"Throughout history, men have worn hats as a way of showing how much better they are than other men. “I buy hats,” a behatted man seems to say. “I am better than you.”
In wartime, hats were a useful way of conferring rank, and ensuring that casualties were confined to the lower classes (hence the famous command of “Don't fire till you see the tops of their heads” at the Battle of Bunker Hill by William Prescott, a general renowned for only shooting enemy combatants who were poor). During peacetime, hats have been instrumental for men to let the non-hatted know just who is wearing the hat around here.\"