Originally developed in 2004, the current iteration of the Source engine was created for use with the titles "Counter Strike" and "Half-Life 2". Since then, through heavily-modded player-created tools and through open-source development from smaller teams, the Source engine has allowed players to truly adapt and create new experiences not only for themselves, but for other players, as well. Mods are widely available through the company's Steam software (which allows for the download of games directly to a person's PC), and are often free. Source boasts "sophisticated character animation, advanced AI, real-world physics, shader-based rendering,", according to Valve's official site.
The Source engine aids in providing better facial animation for in-game characters, as well as various improvements and additional support across major platforms. The Source engine is also adaptable for both PC and console releases, with "Portal" and its sequel hitting both PS3 and Xbox 360, as well as both entries in the "Left 4 Dead" saga. According to Valve's official site, "We don't like to brag, but Source is considered the most flexible, comprehensive, and powerful game development environment out there."
With this new engine currently in development, this could indicate why "Half-Life 3", the long-awaited third entry in Valve's storied science fiction first-person shooter franchise has been on hold for so long. What better way to launch a new Source engine than with a game destined to be a monumental success with fans and critics alike? If "Half-Life 3" is the Source 2 engine's coming out party, Valve will outdo previous entries in the series by a wide margin.