OSSEC – Open Source Host-based Intrusion Detection System
http://www.ossec.net/

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. A list with all supported platforms is available here. OSSEC is a free software and will remain so in the future; you can redistribute it and/or modify it under the terms of the GNU General Public License (version 3) as published by the FSF – Free Software Foundation. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog. This will be added to Security Resources Internet MiniGuide.