This weekend Call of Duty: Modern Warfare 3 PC is free-to-play. The F2P offer includes multiplayer. The event starts at 1PM Eastern (10AM Pacific) on Thursday the 26th of April. You can begin pre-loading the game on Steam from here. You can purchase the game as a PC download for $40.19 from Gamestop or $35 from Amazon.
Preload now from Steam.