Papervision3D is an incredible open-source library for building 3D applications in Flash. You can create basic 3D objects and animate them with Actionscript, but if you really want to create something cool, you need to import a 3D model made with a program like 3D Studio Max, Maya, or Blender. Several different 3D file formats can be imported into Papervision – the most popular format seems to be