The Source Filmaker was released in August and is an amazing tool for inspiring artists. It helps create endless amount of cool looking in-game clips and movies.
Buried beneath all those files and code, over 60 files hinted toward a "next-gen" engine. Last week marked Valve's founder, Gabe Newell's birthday. Gabe began answering questions and one of them was related to the Source engine and he has confirmed that the team has been working on the new engine for a while now and they're just waiting on a new game to test it out with.
Could it be Half Life 3?