The Source Filmmaker (SFM) is the movie-making tool built and used by Valve to make movies inside the Source game engine, the SFM uses the same assets as the game, so 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. If you’ve been watching the recent Team Fortress 2 videos released from valve on YouTube you should know that they’ve been made using this tool.
It is interesting to also note that the Source Filmmaker apparently does have support for stereoscopic 3D rendering, so you can output stereoscopic 3D movie clips right from it and since you are using 3D models used in a game there is no problem with rendering them in stereo 3D. Currently SFM is in beta stage and if you wan to try it you can apply to get a beta key via Steam and be able to try it out.