For the uninitiated, Blender is an open-source, completely free 3D modeling, animation, rendering, and compositing program. The Blender Foundation does these open movies to drive development of Blender in order to meet the needs of real-world CG projects.
Their last project was "Sintel", which was highly acclaimed (the director now works for Pixar as a direct result of his work on "Sintel"). Before that, "Big Buck Bunny".