If you pre-order Modern Warfare 3 through many Digital Distributors, Call of Duty 4: Modern Warfare free, the game that started it all. Call of Duty 4 is a game that people are still playing today as it is a classic and will always remain a classic with it being balanced, with it being the first game to introduce the Perk system as well keeping killstreaks to a basic number of three (UAV, Airstrike & Attack Helicopter).
While there may be other Digital Retailers participating in the promotion, we know that you can take part in it by pre-ordering Modern Warfare 3 at Steam, Direct2Drive, GameStop, and the Activision Store. This is an international promotion and you also get Call of Duty 4: Modern Warfare right away after pre-ordering so you can play while you are waiting for the November 8th release of Modern Warfare 3.