OSSEC is an Open Source Host-based Intrusion Detection System.
It mixes together all the aspects of HIDS (host-based intrusion detection) and Security Incident Management (SIM)/Security Information and Event Management (SIEM) together in a simple, powerful, and open source solution.
OSSEC’s key benefits are:
OSSEC performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response.
To check the operating systems and log formats that OSSEC supports, please visit their page.