Source Filmmaker application, apparently – savvy code hackers at ValveTime.net uncovered references in the code to a "Source 2 engine," indicating that a second iteration of Valve's "Source" engine is in the works. The references go beyond text, as several tool icons (labeled "Source 2 tools") were also found.
Moreover, the engine is labeled as "next-gen," hinting at what its likely application will be – next-gen video games. Valve has yet to announce any next-gen projects, and the company's never said a word about its next Source iteration.
Half-Life 2, the Left 4 Dead series, and several other Valve games are based on the original Source architecture, dating all the way back to 2004. It's been criticized in recent years for inflexibility and agedness, and a reboot certainly wouldn't hurt. Valve had yet to respond to request for comment as of publishing.