OSSEC is a scalable, multi-platform, open source Host-based Intrusion Detection System (HIDS). It has a powerful correlation and analysis engine, integrating log analysis, file integrity checking, Windows registry monitoring, centralized policy enforcement, rootkit detection, real-time alerting and active response.
It runs on most operating systems, including Linux, OpenBSD, FreeBSD, MacOS, Solaris and Windows. You can view here all the supported operating systems and log formats.
License: GNU General Public License (version 3)