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Blender is a free 3D modeling program. When I went on a tour of the EA offices in Richmond (I won a contest), Blender was recommended to me and the rest of the group by one of the modelers there. He said if we couldn't afford something like 3DS Max (The 3D modeling program used to make every Halo game) we should download it.
I took a class with it and it works pretty well. It's available for Mac and PC.