You can use any 3D modeler that is able to export .3DS files. The only free one that I know of is Anim8or. You can find it at
www.anim8or.com. It is somewhat primative but it is easy to use and people have made some very nice projects with it. Going up a notch you might look into Freespace or Rhino.
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NoLimits Dev Team
http://www.nolimitscoaster.de