Modern Warfare 2 is a direct sequel to Call of Duty 4: Modern Warfare. If you haven't picked up COD4, there's a good reason to do so this weekend.

From now until Monday, the military shooter is 40% cheaper on Steam, Impulse, and Direct2Drive. The discount brings it down from $40 to $23.99. The fact that it still has a normal price tag of $40, despite being two years old, goes to show you how popular it still is.

Modern Warfare 2 arrives on November 10th. I imagine there'll be plenty of folks playing COD4 online even after that date, though.