America's Army (2002/2009)
Platform: PC, Mac OSX, Linux
Publisher: United States Army
Leave it to the world's largest military industrial complex to produce the world's best advergame. America's Army may not be on par with Call of Duty or Battlefield in terms of features, but it's probably the most mechanically authentic first-person shooter on the market, and just like any good recruiting tool, it's made available completely for free.
On its third iteration now, the game is used both as a means to draw young impressionable kids into the military and as a training tool for soldiers running simulations. Unlike most FPS titles, AA forces players to follow proper rules of engagement and penalizes players harshly for breaking them, allowing players to get a small taste of how actual combat takes place in the battlefield.
There's a great deal of controversy over this game, largely around the ethics of promoting military life to children (the game is rated T for Teen). Unlike most advergames that are merely there to sell cheeseburgers or soda, this is selling a job and a lifestyle that many would agree is rather hazardous. It's hard to be definitive in judging the game this way though, because even though it's a recruiting tool, it does a great deal of good educating would-be candidates about the realities of enlisting.