Blender, or XSI Modtool.
Blender 2.49 was just released yesterday. It's free, (and free, as in there is no EULA on what you can do with your creations, unlike the free XSI tools), and doesn't require a lot of resources to use. You just download a simple ~15mb zip file and unzip it to a folder, and it's ready to go as soon as you double click on the .exe. The interface is awkward to get used to, but once you understand it, it flies.
I recommend going to vimeo and watching all the beginner blender tutorials. It's easier than reading the docs at first, because everything will click faster as you watch someone actually work with it. vimeo tends to be better than youtube for video quality.
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