I'd say Valve's Left 4 Dead is a bona fide hit. The four player co-op zombie survival game has sold over 2.5 million copies at retail. This does not include sales generated from Valve's online digital distribution platform, Steam, which recently saw a 3000% sales increase when the game was dropped in price.
These numbers were revealed by Michael Booth, the project lead, during a presentation at GDC 09. Left 4 Dead was released last year to critical acclaim, thanks to the game's strong focus on co-op play and incredibly immersive atmosphere. Valve plans to release a free download pack called the Survival Pack on April 21st for both the Xbox 360 and PC versions of the game.