#4 Team Fortress 2
Team Fortress 2 was originally a part of "The Orange Box", which contained one of the Half-Life 2 Episodes, and Portal. It's been years since its release, but the game's has done nothing but increase in popularity over time thanks to the addition of many new gameplay modes and an item and trading system.
Now completely free to play, players in Team Fortress 2 can equip a variety of outfits, hats, and weapons—most of which are acquired through play. Those with less patience can even buy the items with real money or trade for items with other players.
The game's latest update, titled "Mann vs Machine" adds an all new gameplay mode that's reminiscent of both Gears of War 2's Horde Mode and Tower Defense. Players can choose from one of the game's classes and develop a strategy to defend the base against oncoming waves of evil robots.
