A games involving alcohol, sillyness, rules which serve no purpose except to ensure drunkenness, and (optionally) a DrinkingHat!
Some of GreigMcGill's favourites include:
TreeMan (or ThreeMan), WhizzBoingBounce?, and the ElectionNightDrinkingGame.
Other examples include CircleOfDeath and the Jakob Nielsen Drinking Game.
If you drink too much you can always take the BeerScooter home.