Team Fortress 2 is a free-to-play first person shooter (FPS) game developed by Valve, based on the Source engine (developed by Valve), and released originally on the Orange Box on October 10, 2007. It was released by itself in stores on April 9, 2008 for Windows computers, and a Mac OSX port followed two years later. Although the Orange Box was not released to the PlayStation 3 console, TF2 was ported there on November 22, 2007, to rival the 360 version of the game. However, most of TF2's sales have come from Steam. It was led by John Cook and Robin Walker, two profound developers at Valve. This game is the sequel to Team Fortress, originally a mod for Quake, but it was released as a standalone game later on. Although it was normally $20 to $30 USD retail, on June 23, 2011, Team Fortress 2 became free-to-play on the Steam platform. The game is now supported through microtransactions through the store in-game.