Argus

Argus was originally designed to monitor servers and network connections in a mission-critical ISP (Internet Service Provider) environment, and scales well from small-businesses through large enterprises. It will monitor nearly anything you ask it to monitor
(TCP + UDP applications, IP connectivity, SNMP OIDS, Programs, Databases, etc).

Argus was originally designed to monitor servers and network connections in a mission-critical ISP (Internet Service Provider) environment, and scales well from small-businesses through large enterprises.

It is open-source available at no charge.
It has a clean and intuitive web interface.
The web pages can easily be understood by non-technical people.
It can generate graphs of what is going on.
It can monitor network connectivity (Ping test)
It can monitor TCP/UDP ports
It can monitor a wide variety of TCP/UDP applications (HTTP, SMTP, RADIUS, ...)
It can monitor the output or exit code of a program (Program test)
It can monitor the content of a web page (such as a shopping cart application)
It can monitor the authoritativeness of a nameserver
It can monitor SNMP OIDs (such as BGP status, UPS voltage, room temperature, ...)
It can monitor the results of SQL queries
It can monitor itself.
It can be extended to monitor things that the author didn't think of
It can notify someone (or many people) when something happens
It can escalate, and notify someone else, if things don't get fixed.
It can not alarm for known downtime (maintenance overrides)
It will summarize and rate-limit multiple notifications to prevent paging-floods.
It keeps historical statistics, for analysis or SLA verification.
It scales well and can monitor many, many things.
It can restrict users to viewing only certain items (user "views")
It can restrict users access to certain features (access control)
It can support IPv6.
It can support SNMPv3.
It can support l10n for your native language.
It can support redundant multi-server configurations.