Rendering models using a variety of materials allows a finished object to be viewed and analyzed before committing to cutting wood or printing plastic. SimLab Composer works very well with Alibre Design. Here is an example showing my fan mount for the current RepRapPro Mendel design:

The nozzle mount is an STL from RepRapPro. The fan is an IGES file from GrabCAD. The M3 nuts and bolts are STLs from TraceParts. Finally the fan mount was exported as 3D PDF from Alibre Design.