Valve has just released the Source Filmmaker on Steam, which is now available for free as a public beta.
The Source Filmmaker (SFM) is the movie-making tool that Valve developed in-house, and has used to develop all of its animated films for Team Fortress 2. The SFM makes use of all the in-game assets available to the Source game engine and allows the user to direct a film with a full range of controls.
The SFM uses the hardware rendering power of the modern gaming PC and provides its users with a “what you see is what you get” environment so you can iterate and make changes on the fly, in the context of what the film will look like for the audience.
The SFM public beta comes with all the base game assets from Team Fortress 2 as well as additional assets from the “Meet the Team” short films. Valve has also set up a host of tutorials and an official wiki to get you started.