OK, this Help Net Security article on #opensource security tools is pretty good.
I usually cringe at articles like this, that invariably trot out the ‘ole standbys like Wireshark, Tor, Nmap, and ClamAV… but this list is actually pretty fresh and helpful. Give it a look.
https://www.helpnetsecurity.com/2025/01/27/open-source-cybersecurity-tools-free/