Garry's Mod breaks ten million copies sold.

Garry Newman thanked the fans through his Twitter, stating that GMod sold "its ten millionth copy over new years." Garry's Mod started out as a simple mod for Valve's Half-Life 2, then it was released as a standalone game in 2006, selling six million units in eight years, with the following four million copies sold in a little over a year.

To me what's most attractive about Garry's Mod is its sandbox nature, giving you complete freedom to do whatever you want.