OSSEC (Open Source SECurity)

OSSEC  (Open Source SECurity) is an host based intrusion detection system. Its primary function is to monitor Linux systems (files, configurations, rootkits, process monitoring) and alert to any changes. OSSEC is derived from TrendMicro, and the software can be found on enterprise systems monitoring critical hosts such as web servers, database backends etc.

More information can be found at OSSEC