Datadog!

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.

NetWorx is a simple and powerful tool that helps you objectively evaluate your bandwidth situation. It can ensure that you do not exceed the bandwidth limits specified by your ISP, help identify sources of network problems, and help track suspicious network activity. The software features highly customizable visual and sound alerts that can alert you of a variety of events, such as bandwidth quotas, network downtime, and suspicious network activity. The traffic is represented on graphs as well as logged to a file, so you can view and save statistics about your daily, weekly and monthly bandwidth usage at any time. NetWorx became commercial with version 6.0.0 released January 1st, 2017; the last free version is 5.5.5. »

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.

Icinga is a fork of Nagios and is backward compatible. So, Nagios configurations, plugins and addons can all be used with Icinga. Though Icinga retains all the existing features of its predecessor, it builds on them to add many long awaited patches and features requested by the user community. Icinga is an enterprise grade open source monitoring system which keeps watch over a network and any conceivable network resource, notifies the user of errors and recoveries, and generates performance data for reporting. Scalable and extensible, Icinga can monitor complex, large environments across dispersed locations. Icinga is licensed under GPL V2. »

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.

Prometheus is an open-source systems monitoring and alerting toolkit originally built at SoundCloud. Since its inception in 2012, many companies and organizations have adopted Prometheus, and the project has a very active developer and user community. It is now a standalone open source project and maintained independently of any company.

PHP Server Monitor is a script that checks whether your websites and servers are up and running. It comes with a web based user interface where you can manage your services and websites, and you can manage users for each server with a mobile number and email address.

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.

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.