The list includes nine games total including Half-Life, Counter-Strike and The Orange Box. These figures include retail sales only, meaning any copies moved via digital distribution aren't accounted for in these figures.

Valve's digital distribution platform, Steam, released in 2003, is sure to have helped move hundreds of thousands of copies of their games, if not more, which would account for the rather noticeable gap between Half-Life and Half-Life 2 sales.

Earlier this year Valve founder and front man, Gabe Newell, predicted that sales via Steam of Valve's own products would surpass that of their retail sales.

• Half-Life (1998) -- 9.3 Million
• Half-Life: Opposing Force (1999) -- 1.1 Million
• Half-Life: Blue Shift (2001) -- 800,000
• Counter-Strike (2000) -- 4.2 Million
• Counter-Strike: Condition Zero (2004) -- 2.9 Million
• Counter-Strike: Source (2006) -- 2.1 Million
• Half-Life 2 (2004) -- 6.5 Million
• Half-Life 2: Episode One (2006) -- 1.4 Million
• The Orange Box (2007) -- 3 Million

It's important to note that sales of Counter-Strike (2000) and Counter-Strike: Source (2006) represent retail sales of the stand alone game only, it doesn't account for any sales through digital distribution or the consumers who received the mod for free with the original Half-Life, Half-Life 2, The Orange Box or any other package.

Combined, the figures published by Valve total 32.8 million units sold, which is increased to 36.5 million if Left 4 Dead projections are included. Counter-Strike on the Xbox sold an additional 1.5 million units.