OSSEC is an open source Host-Based Intrusion Detection System (HIDS). It has a powerful correlation and analysis engine that integrates log analysis, file integrity monitoring, centralized policy enforcement, rootkit detection, real-time alerting, and active response.
It provides a pretty complete coverage if you are looking for endpoint (server) monitoring.
If you have not used OSSEC before, I recommend reading this guide to get started:
More details here: