Zenoss is an enterprise-class open source server and network monitoring solution that distinguishes itself through its virtualization and cloud monitoring capabilities, which are frequently difficult to find in older free monitoring tools.
2. OSSIM Full-fledged SIEM functionality doesn't have to blow the budget. OSSIM, or Open Source Security Information Management, serves up a package of open-source detection tools paired up with a correlation engine meant to minimize your heavy lifting in the analysis department.
3. Spiceworks Spiceworks is a solid general IT network and systems management engine with a load of security monitoring capabilities, and gives small organizations the ability to monitor applications and systems and provide alerting to head off extended issues.
4. Splunk Splunk is a log collection and analysis tool that gives administrators the power to condense their monitoring data for applications, network devices, servers and more in order to crunch it and put out actionable reports. The free version, however, is limited to 500 MB per day of data.
5. Zabbix You can track network devices and services, servers, and applications for traditional security logging and analysis, plus performance management and capacity planning using the open source package Zabbix.
6. FireGen Analyze firewall logs for major vendors such as Cisco, Juniper, SonicWall, Fortigate, and Symantec using FireGen, which provides the ability to identify and explain events recorded in order to more easily act on the data's meaning.
7. LANSweeper You can't monitor assets you don't know about. Gain better visibility into what hardware and software assets sit on your network using LANSweeper, an open source and agentless scanner that can help you lay the groundwork for better monitoring practices.
8. Nmap An oldie but a goodie, Nmap presents administrators in charge of heterogeneous networks the capabilities to track network inventory, manage service upgrade schedules and monitor host or service uptime.
9. Nagios Another fully featured open source network management and monitoring tool, Nagios will keep tabs on network services, host resources, network probes. The tool can monitor via remotely run scripts and supports encrypted tunnels for remote monitoring.
10. OSSEC HIDS Compatible with the full range of operating systems, this open source OSSEC HIDS host-based intrusion detection system will do log analysis, integrity checking, Windows registry monitoring, and rootkit detection. It's equipped with time-based alerting and active response to offer maximum proactive action.