IT Security/Threats/Threat Mitigation/Intrusion Detection Systems

An intrusion detection system (IDS) is a device or software application that monitors a network or systems for malicious activity or policy violations. There are some free and open source products available on the market such as:
 * ACARM-ng
 * AIDE
 * Bro NIDS
 * Fail2ban
 * OSSEC HIDS
 * Prelude Hybrid IDS
 * Sagan
 * Samhain
 * Snort, GPLv2+ developed by Cisco.
 * Suricata