OSSEC

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 2) as published by the FSF - Free Software Foundation... OSSEC is a multiplatform, open source and free Host Intrusion Detection System (HIDS). You can tailor OSSEC for your security needs through its extensive configuration options, adding custom alert rules and writing scripts.

Also known as: OSSEC Foundation