The Game of Thrones mod for Crusader Kings II is a free 'reskinning' of the original game which allows you play in Westeros instead of medieval Europe. The mod also features original artwork and music, and some comprehensive engine changes to account for how life works in the Seven Kingdoms.
PC Gamer is running a diary of a campaign in this game. Even if you're not interested in the game, it's worth a read as the player tries to play as a 'more sensible' version of Ned Stark and immeditely runs into unforseen events that cause total chaos. An entertaining read. Part 1 of the diary is here, Part 2 here and Part 3 here. Further updates to the diary will follow every Sunday until it is completed (or Ned dies, which is an occupational hazard).