OSSEC
OSSEC stands for Open Source HIDS Security. It is entirely free to use. As a host-based intrusion detection system, the program focuses on the log files on the computer where you install it. It monitors the checksum signatures of all the log files to detect possible interference. On Unix-like systems, it will monitor any attempts to get to the root account. Although OSSEC is an open-source project, it is owned by Trend Micro, a security software producer.