Blender is the open source software for 3D modeling, animation, rendering, post-production, interactive creation and playback.
I realize that whole days go by when most people don't think about creating 3D images or movies with their computer, but if the thought has ever crossed your mind, or if you ever wanted to try your hand at it, then you can't go wrong giving this a try.
Blender works with Windows, Linux and Mac OSX.