Valve has just announced its decision to release the tool it uses to make the animated shorts for Team Fortress 2. It's called the Source Filmmaker, and Valve has just opened the sign-ups for anyone who wishes to use it.
The Source Filmmaker, or SFM, uses the assets of Source games. Anything that can be used in the game can be used in the movie. SFM allows storytellers to work in a "what you see is what you get" environment to mess around with their films in real time, without ever having to switch applications or render each take into a hard copy.
The announcement of the Source Filmmaker is made in conjunction with the release of "Meet the Pyro", the ninth installment of the Team Fortress 2 "Meet the Team" series. It too, Valve notes, was made with the SFM.
You can sign up for the SFM beta at the official website, which is a little slow to load right now due to traffic.