NOTE: Half Life 2's numbers are not necessarily accurate- most of that game's sales came before Steam began compiling user data (which is what this is based on), so its actual sales should be significantly higher).
Also, I am absolutely stunned at that sales figure for Civilization V- 6 million sold so far? For reference, the entire Civilization series up to IV had sold 12 million combined. So glad that such an amazing game got amazing sales.
Anyway, here's your source.