Snort - Snort is the foremost Open Source Intrusion Prevention System (IPS) in the world. Snort IPS uses a series of rules that help define malicious network activity and uses those rules to find packets that match against them and generates alerts for users.
Suricata - Suricata is a high performance, open source network analysis and threat detection software used by most private and public organizations
Zeek - Zeek is a passive, open-source network traffic analyzer.