OSSEC HIDS Version 1.2 is available!
OSSEC is an Open Source Host-based Intrusion Detection System. It performs log analysis, integrity checking, Windows registry monitoring, rootkit detection, real-time alerting and active response.
It runs on most operating systems, including Linux, OpenBSD, FreeBSD, MacOS, Solaris and Windows.
Free download at: http://www.ossec.net/en/downloads.html