Valve's zombie co-op shooter Left 4 Dead was certainly a big departure from the Half-Life universe that the developer had created before but it paid off in spades. According to a Shacknews story, Valve's Michael Booth revealed in a GDC panel this week that the game has sold 2.5 million copies worldwide.

That's a lot for any game (and that figure includes both PC and Xbox 360 numbers) but that may only be part of the story. As Shacknews states, the 2.5 million copies mentioned are only for retail sales. It does not include the sales of the PC version that have been made directly via Valve's Steam service. Valve is pretty notorious for not including specific sales figures for their games that have been sold via Steam.