The Animatrix is a series of several short animated stories that tell the back-story of the Matrix. The strongest pieces are at the beginning, and the ones at the end are just okay. But the first few are absolutely amazing. It is truly a beautiful work of art, well-written and wonderfully rendered. It is rich, bold, imaginative, frightening, and enlightening.
I kind of figured that if it were really any good, it would have been in the theater, instead of going straight to video, especially with all the hype about The Matrix: Reloaded. But in fact, The Animatrix is simply one of the best movies I've ever seen.