Spiral knights
Son of a Nutcracker! - Hit an opponent with a snowball in Lockdown.
Psychonauts
Holiday Dinner - Cook and consume two different kinds of roast in one sitting.
Dungeons of Dredmor
Diggle Nog! - Make some Diggle Nog. Who doesn't love a nice cold glass of Diggle Nog?
Red Orchestra 2
Fighting The War on Christmas - Participate in a successful territory capture on Commissars House
Tropico 4
Megalopolis - Reach a population of 1000
AI War
In The Midst Of A Cold, Dark Winter - Lose at least 30,000 ships in a single game.