Today, Valve launched the highly anticipated first-person puzzler Portal 2, and the game appears to have critical success well in hand. While Portal 2's commercial performance remains to be seen, it will be attempting to fit into a pair of oversized shoes, as Valve confirmed for GameSpot today that, even after excluding Steam sales, the original Portal has to date sold nearly 4 million units worldwide.
Portal's sales milestone comes despite humble beginnings. The game began life as the senior project of a group of students at the DigiPen Institute of Technology, titled Narbacular Drop. After winning the Independent Game Festival's Student Showcase Award in 2006, the game and its creators were subsequently signed by Valve.
Narbacular Drop's interconnected portal concept was adapted for Valve's Half-Life universe and renamed, aptly enough, Portal. The first-person puzzle game was then packed in as part of Valve's 2007 blockbuster The Orange Box, which also included Half-Life 2, that shooter's first two episodes, and the multiplayer-only Team Fortress 2.
Portal quickly proved to be the standout release of the package, winning such accolades as the 2008 Game Developer's Choice Awards' Game of the Year.
For more on Portal, check out GameSpot's recent interview with writer Erik Wolpaw.