Angel is a simple yet useful tool to monitor the services on your network. Technically speaking, it's a Perl program that runs every 'n' minutes (usually fired from your cron) and calls different perl subprograms (referred to as "plugins" from now on) to do the actual testing. It will then generate an HTML table containing the status of your network.
Lrp Network Monitor is an applet/application that displays information about network devices on a Linux router. This information can be displayed either in plain text, or in graphical format (either bar-charts, line-charts, or a histogram). It also displays the status (online/offline) of ISDN-devices, and can trigger actions like dial or hangup.
Very Simple Network Monitor is a bash shell script CGI that monitors a number of services on a network. It currently checks for node-up, TCP service up, and UDP service up. A service or system can be put in maintenance status to avoid repetitive alarms. It is easy to customize, trigger alarms by email, or whatever you like (depends on nmap for TCP/UDP service checks). Installation is also simple. It generates HTML with all statuses, and for all services just in one view.
NEPM monitors and reports uptime, critical events and their predecessors, access rates, bytes-served rates, and error rates for network node equipment. Hardware and software elements within the nodes are tracked and reported separately to make possible rapid fault isolation. It is a very general, highly configurable, two-part software system that captures and analyzes logged performance data from IP-networked equipment and reports it via email and Web pages. Current conditions and history from systems based on Windows NT/2000, Unix, and Unix-style operating systems can be tracked and reported. Most major server, switch and router systems can be monitored, without running agents on the target systems. NEPM itself is system-independent and can be hosted on either a Unix or Win NT system or a combination of these with equal ease.
SkipoleMonitor is a free network monitor. On running the program, a GUI window appears, and hosts can be added, which Skipole Monitor will regularly ping, showing Red-Yellow-Green status results via a built-in Web server. Email and syslog alerts can be generated as hosts change status.
netstat-monitor is a commandline tool for monitoring network connections. Its output is similar to the output from the netstat command with the options "netstat --inet -alp". Netstat-monitor can be left running, though, and will report new connections as they are made. Also, filters can be created to limit what's displayed to just what's unexpected or interesting.