If your MCAD software of choice is Inventor, Solid Edge, SpaceClaim, or Alibre Design, then SYCODE has a free OpenRP [rapid prototyping] plug-in for you. I'll let SYCODE describe them:
The free software offered by SYCODE under the OpenRP Initiative is designed to help the RP industry overcome the limitations of the STL file format - large file size and zero security. Using this software CAD software developers can add RP file support to their CAD systems, end users can exchange their designs quickly and securely and service bureaus can accept RP files from end users to quote and build prototypes.
Download from here.
The plug-in was already available for AutoCAD, SolidWorks and Rhinoceros; the company is writing plug-ins for additional CAD packages.