Zenoss Core!

An uncompromising mature enterprise-level platform designed for real-time monitoring of millions of metrics collected from tens of thousands of servers, virtual machines and network devices, effortlessly scaling to even larger environments.

Join millions of IT professionals and download Spiceworks, a free network monitoring software with network management tools, help desk ticketing app, network mapper and more.

Datadog is a monitoring service for IT, Dev & Ops teams who write and run applications at scale, and want to turn the massive amounts of data produced by their apps, tools and services into a unified view of IT infrastructure. Datadog's capabilities are provided on a SaaS-based data analytics platform that enables multiple teams to work collaboratively on infrastructure issues.

LibreNMS is an autodiscovering PHP/MySQL based network monitoring system forked from Observium. LibreNMS aims to be easy to use, painless to deploy, and support monitoring of a wide range of devices. Observium's license changed to a modified QPL license in May 2012, and recently was split into free and commercial versions. Our aim with LibreNMS is to continue to create a usable NMS, but with a community focus and GPL licensing.

An advanced, easy-to-use monitoring solution for your entire network. The software's features include: up/downtime monitoring, traffic and usage monitoring, packet sniffing, failover clustering, in-depth analysis and concise reporting. A user-friendly web-based interface allows users to quickly auto-discover and configure the network devices and sensors they wish to monitor. All common methods for acquiring network usage data are supported: SNMP and WMI, Packet Sniffing and NetFlow / sFlow / jFlow. It also offers SMTP/IMAP Roundtrip-Email Monitoring and VMware Monitoring, VoIP Monitoring, Toplists for Packet Sniffing and Netflow Sensors, as well as sensor aggregation using its built-in Sensor Factory. PRTG Network Monitor includes more than 200 sensor types for all common network services (e.g. PING, HTTP, SMTP, POP3, FTP, SSH etc.), allowing users to monitor networks for availability, speed and failures. As soon as an outage occurs, alerts are sent via email, SMS, pager messages and other means. Request times and downtimes are recorded in an internal database, making it easy to compile performance, downtime and SLA reports. Monitoring, reporting and notifications can be automatically scheduled at specific times. High Availabiltiy is guaranteed by the innovative failover cluster, and remote probes can be used for easy multi-location monitoring. Network maps can be created which provide users with a unique visual network management tool. PRTG Network Monitor's Freeware Edition is completely free for personal and commercial use. The Commercial Editions are required if you want to monitor more than ten sensors. »

Munin is a networked resource monitoring tool that can help analyze resource trends and "what just happened to kill our performance?" problems. It is designed to be very plug and play. A default installation provides a lot of graphs with almost no work.

Cabot is a free, open-source, self-hosted infrastructure monitoring platform that provides some of the best features of PagerDuty, Server Density, Pingdom and Nagios without their cost and complexity.

Observium is an autodiscovering PHP/MySQL/SNMP based network monitoring which includes support for a wide range of network hardware and operating systems including Cisco, Linux, FreeBSD, Juniper, Brocade, Foundry, HP and many more.

Pandora FMS is an enterprise-ready monitoring solution that provides unparalleled flexibility for IT to address both immediate and unforeseen operational issues, including infrastructure and IT processes. It uniquely enables business and IT to adapt to changing needs through a flexible and rapid approach to IT and business deployment.

Check_MK is comprehensive IT monitoring solution in the tradition of Nagios. Check_MK is available as Raw Edition, which is 100% pure open source, and as Enterprise Edition with a lot of additional features and professional support.

Cacti is a complete frontend to RRDTool, it stores all of the necessary information to create graphs and populate them with data in a MySQL database. The frontend is completely PHP driven. Along with being able to maintain graphs, data sources, and Round Robin archives in a database, cacti handles the data gathering. There is also SNMP support for those used to creating traffic graphs with MRTG.

OpenNMS is an enterprise grade network monitoring and network management platform developed under the free software or open source model. It consists of a community-supported, free-software project as well as an organization offering commercial services, training and support.