How evergreen, City 17

Gamasutra notes that Game Informer magazine has run life to date retail sales for Valve's Half-Life franchise, and even not counting units sold through its Steam service (keeping those figures traditionally tightly held), the numbers are duly impressive:

Half-Life - 9.3 million Half-Life: Opposing Force - 1.1 million Half-Life: Blue Shift - 800,000

Counter-Strike - 4.2 million
Counter-Strike: Condition Zero - 2.9 million
Counter Strike: Source - 2.1 million
Counter-Strike [Xbox] - 1.5 million

Half-Life 2 - 6.5 million
Half-Life 2: Episode One - 1.4 million

The Orange Box - 3 million

Left 4 Dead - 3.6-3.9 million (projected)

As Gamasutra points out, while the figures on their face would suggest a downward trend in sales for each new volume of the game, the 2004 release of Half-Life 2 was the first to be released simultaneously at retail and via Steam.

Analysis: Valve's Lifetime Retail Sales For Half-Life, Counter-Strike Franchises [Gamasutra]