CrowdSec
MIT-licensed collaborative intrusion prevention that blocks malicious IPs from your logs and shares the signals network-wide, so every user gains herd immunity. Premium blocklists are paid.
Threat Detection
Category: Threat Detection under Security & Privacy
Security Onion
Source-available (ELv2) network security monitoring on Suricata, Zeek, and the Elastic Stack. Self-hostable and airgappable; SSO and disk encryption require the paid Pro tier.
OPNsense
A FreeBSD-based firewall and router that replaces Fortinet and Cisco appliances, with IDS/IPS, VPN and traffic shaping built in. The paid edition only gates a hardened, slower release track, not features.
Wazuh
GPL-2.0 unified XDR and SIEM platform from Wazuh, Inc. — all capabilities including RBAC, threat hunting, and compliance mapping modules included in the open-source binary; paid tier is managed cloud only.