I have been writing engineering design software for fifteen years, in various disciplines. I've written electrical, mechanical, and energy auditing programs. Most are featured in my textbooks from McGraw-Hill. I have always offered them for free, and many people have benefited.

The programs are unique to the industry because they are intended for non-professionals. They do all the robust engineering calculations but the front end is carefully designed to make the programs easy to learn and use. The energy analysis programs especially have all the data and tables that are needed to do a full engineering analysis.