This afternoon I played around with Blender a bit. It's a free and open source, cross-platform 3d modeling and animation program. I was able to follow this tutorial and make a simple model of a person. If you want to jump right into animation, check this one out. If you're willing to spend a couple of hours getting familiar with the user interface and moving around in 3 dimensions, you could make some pretty cool stuff with this. There's a whole animation festival with entries that all come from Blender.