This is a program made by ASMI and modified by Ronso in this thread. This is a genius free LISP Program and can be used in many trades, but has most usefulness in the HVAC, Mechanical, and Piping industries for drawing 2D CAD designs. It allows you to create Duct, Pipe, and Segmented Duct or Piping with automatic elbow or fittings to accurate dimensions via some initial input at the command prompt.
The programme in action
Function Syntax: DUCT
For instructions on how to run the program see here.