Robin Good: The Source Filmmaker (SFM) is a story-telling movie-making ecosystem built around Valve Source Game Engine.
SFM allows the user to tap into all of scenes and components of an existing 3D game, to create, re.animate, and re-combine characters, movements and plots in any imaginable way.
"The SFM allows a storyteller to create an entire movie on location inside the video game world of Team Fortress 2. Movies exported with the SFM can be uploaded to the web or shared like any normal movie file."
Unlime most other 3D cinematographic tools which typically take care of only a part of the movie production (renderer, 2D video clip editor, a keyframer, a motion capture editor, a sound editor, etc.) the SFM is a hybrid of all these workflows merged into a single system, where you can create an entire movie by reusing assets and events from the video game world.
"Because the SFM uses the same assets as the game, anything that exists in the game can be used in the movie, and vice versa. By utilizing the hardware rendering power of a modern gaming PC, the SFM allows storytellers to work in a what-you-see-is-what-you-get environment so they can iterate in the context of what it will feel like for the final audience."
You can apply for a beta key here and install it from Steam like any other free-to-play game. Note that the beta key request requires Steam to be running on your PC.
More info: http://www.sourcefilmmaker.com/