Part of Half-Life's success can be attributed to the phenomenally popular Counter-Strike, which originally appeared as a free Half-Life mod. Sold separately, Counter-Strike sold 4.2 million units at retail in its Half-Life-derived incarnation, while the updated Counter-Strike: Condition Zero and rebuilt Counter-Strike: Source sold 2.9 million and 2.1 million units respectively. The Xbox port of the game sold 1.5 million units.
Half-Life 2, released in 2004, has sold 6.5 million units at retail. The 2006 followup Half-Life 2: Episode One sold 1.4 million units at retail. Half-Life 2 marked the beginning of Valve's practice of releasing its titles simultaneously at retail and via its Steam digital distribution service.
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