This will only be of interest to amateur radio enthusiasts and electrical engineering folks, but some of the explanatory pages seem to be falling off the web, so I thought I would mention it. EZNEC Pro+ v. 7.0 Antenna Software by W7EL is available for free:
https://www.eznec.com/

Some explanatory pages in the Wayback Machine about why the program is being retired with no support and why there won't be any open source follow-ups:
https://web.archive.org/web/20210927201245/ht … /retirement.htm
https://web.archive.org/web/20210927183438/ht … source_code.htm

Some other free alternatives with different strengths and weaknesses:
https://www.qsl.net/4nec2/
https://www.cenos-platform.com/community-licence (free for non-commercial use)
https://hamsoft.ca/pages/mmana-gal.php (free for amateur radio use)