Guess what, WWII shooter fans? Time for a little excitement. Activision today sent Call of Duty: United Offensive off to retail stores, finally unleashing a second dose of one of last year"s best PC action experiences, and one of the better WWII shooters out there. Taking the reigns from Infinity Ward, the good folks over at Grey Matter Studios have put together the official expansion to Call of Duty, offering a generous helping of new missions, new maps, and new multiplayer modes.