I'm using Amapi 5, EmpressZario. You can get version 4 for free by following the link in the banner in the Amapi forum. Other free packages people use are Wings and Anim8or. Truespace seems to be popular too. Just about any 3D modeling program that can export in a Poser acceptable format (usually 3DS or OBJ) works. The interfaces and methods of use vary so much, it may be best to try all the free versions you can find to see what you work best with.