Nowadays, the heavy reliance on computer networks necessitates minimizing outage time, increasing the availability of services, and preventing network related problems. Such realization requires continuous monitoring and observation. This is not a trivial task. Thus, automatic network monitoring tools are deployed to monitor and analyze the traffic trespassing network devices. There is an increasing demand for automated network monitoring tools and selecting a suitable candidate can become a challenging task. Some computerized network monitoring tools and systems are available, including expensive proprietary/closed-source solutions and Free Open Source Software (FOSS) systems. Three of the most popular FOSS network monitoring systems are: Nagios, OpenNMS and Zabbix. They are solid competitors to the available proprietary solutions. This paper evaluates the strengths and weaknesses of these tools. A qualitative and quantitative evaluation of the tools were conducted through monitoring real-time network traffic. The paper presents a thorough comparison between the tools. The comparison results are vital for network administrators wishing to adopt the studied monitoring tools.