Terragen is an awesome tool for creating photorealistic scenes and imaginary landscapes, with superfly rendering.
Clayscape 3D, as you might have guessed, is a modelling tool which is just like playing with virtual clay, pushing, pulling, sculpting and moudling to create 3D shapes and models.

• Jarfish Clayscape 3D (Mac only)
  FULL PROGRAM!
  

  ClayScape3D is a 3D modelling program which allows the user to interactively sculpt virtual clay in real time. Requires Apple's OpenGL extensions included on the CD.

• Planetside Software terragen (PC only)
  FULL PROGRAM!
  

  Terragen is a photorealistic scenery generator rendering software.