With an auspicious beginning at the Independent Games Festival Student Showcase in 2006, a small game -- named Narbacular Drop -- from a group of DigiPen students went on to inspire an industry darling. Several oft-quoted lines of dialog and millions of "The cake is a lie" jokes later, Portal was a resounding critical success when it launched inside The Orange Box, also home to Half-Life 2, its two follow-ups, and Team Fortress 2.

You knew about the acclaim, but you may not have known that it has moved approximately four million copies in the past four years. What's really surprising, however, is that Valve reached that sales mark without including the numbers for Steam -- numbers that would muddle the accuracy of that four million in terms of actual sales, given that Valve has offered the game free through Steam at various times.

It's also unclear whether that number includes individual copies of Portal sold at retail, the Xbox Live Arcade release (Portal: Still Alive), or any other sales channels. We've reached out to Valve for clarity on the total sales of Portal, but for now, four million is nothing to sneeze at -- especially considering the game's humble origins.