EQUINOX-3D has the first fully interactive ray-tracer in a freely or commercially available 3D application (that I know of).
(Check out the Image gallery).

• Multi-threaded ray-tracer (runs about twice as fast on dual-core CPUs and 4 times faster on quad-cores).
  

• Renderers are fully integrated:
  You can run your own renderer, or even a whole video game (plugins can define renderers) in a normal EQUINOX-3D window.
  For example, it lets you model with the interactive ray-tracer, and:
  - select and unselect things (see the ruby and the cube below)
  - edit 3D models.
  - move objects, the camera, lights, change materials etc.
  Thanks to smart updates, selecting / unselecting is just as fast on the ray-traced view as on any other one!

  


• Interactive option for the render-preview window:
    When you move, rotate objects, lights, change materials, move the camera
    (in the top-right window), the ray-traced view will change interactively.
    This scene renders ~3-100 frames/s on an AMD-64 5600:
    (resolution: 360x270, ray depth: 7)
    
  


• Powerful and fast cross-renderer shader interface (ray-tracer, Cg etc.).
  Per-pixel normals, geometric tangents, texture-space tangents, pre-sampled/shadow-tested area light samples etc. are easily accesible by shaders.
  Source code examples included:
  - Phong with Fresnel-modulated reflectivity and specular highlight
  - Anisotropic shader
  
  Minimal effort needed to implement a shader for both the ray-tracer, the Cg renderer etc. (e.g. the Cg framework automatically generates Cg shaders from a minimal template).
  Identical look for Cg and ray-tracer outputs (within the limitations of graphics hardware):
  

  Ray-tracer

  Cg

  
  
• Multi-texturing
  For example, you can have separate textures for the ambient, reflective etc. colors, or for any parameter of a custom shader:
  
  



• Bump / normal-mapping
  
  



• Area lights
  Any 3D object can be a light source and area lights cast correct soft shadows:
  



• Programmable texture-mappers
  In addition to the built-in mappers (planar, UV, cylindrical etc.) you can write your own mapper that fits a particular object best.
  
  
• Direct support for mirror-ball photograps and angle maps as textures/environment maps
  


• Textured area lights and high dynamic range texture support
  32-bit floating-point values per R, G, B, A channels
  
  


• Colored reflections and refractions
  


• Volumetric absorption (e.g. 'muddy water' effect)
  


• Anti-aliasing with many options
  


• Customizable progressive refinement:
  ->->
  


• Multiple lens types:
   Perspective, Fish-eye, rear-view fish-eye, Cylindrical, Panorama and Angle-map
   (great for creating reflection-maps and illumination-maps)
   
  

• High Dynamic Range (HDR) image support:
  Allows you to save the rendered image with its full color range, as 32-bit floating-point values per R, G, B, A channels.