GKrellM is a single process stack of system monitors which supports applying themes to match its appearance to your window manager, Gtk, or any other theme.
2. BIG BROTHER
Big Brother, web-based solution for IT infrastructure monitoring and diagnostics. Get real-time monitoring for any server.
EtherApe is a graphical network monitor for Unix modeled after etherman. Featuring link layer, ip and TCP modes, it displays network activity graphically. Hosts and links change in size with traffic. Color coded protocols display. It supports Ethernet, FDDI, Token Ring, ISDN, PPP and SLIP devices. It can filter traffic to be shown, and can read traffic from a file as well as live from the network.
Nagios is a powerful monitoring system that enables organizations to identify and resolve IT infrastructure problems before they affect critical business processes.
Designed with scalability and flexibility in mind, Nagios gives you the peace of mind that comes from knowing your organization's business processes won't be affected by unknown outages.
Nagios is a powerful tool that provides you with instant awareness of your organization's mission-critical IT infrastructure. Nagios allows you to detect and repair problems and mitigate future issues before they affect end-users and customers.
The Tkined (Tcl/tK based Interactive Network EDitor)  is a specially constructed wish-based editor. It comes with varied world and regional maps, geographical locations of some IP addresses, a collection of bitmapped images for representing components of a network, and much more. It has a collection of background daemon scripts to run scotty processes as slaves, and a job management for use of those scripts. As such, it can not only use SMTP for network management, but it also can do simple things, like monitoring ping times. [Progress Development]'s valuable "Getting Started with Tkined"  is strongly recommended.
You have a router, you want to know what it does all day long? Then MRTG is for you. It will monitor SNMP network devices and draw pretty pictures showing how much traffic has passed through each interface.
Routers are only the beginning. MRTG is being used to graph all sorts of network devices as well as everything else from weather data to vending machines.
MRTG is written in perl and works on Unix/Linux as well as Windows and even Netware systems. MRTG is free software licensed under the Gnu GPL.
RRDTool. This page details the use of RRDTool for monitoring the network traffic through one or more network interfaces.
PIKT is cross-categorical, multi-purpose software for monitoring and configuring computer systems, administering networks, organizing system security, and much more.
PIKT is intended primarily for system monitoring, and secondarily for configuration management, but its versatility and extensibility evoke many other wide-ranging uses.
Autostatus is a network monitoring program for hosts and services. It is designed to befast and extremely easy to use. It monitors by icmp pings and byattempting TCP connections to specified ports.
bcnu is a web based system management tool which delivers information on the status of networked systems in a simple and easy to use manner. Have a look at the sample screens to see how easy it is. It uses a web browser to display information about hosts in a tabular form. Coloured icons show the status of monitored conditions and clicking on these will bring up detail about the state of the system. Historical information can be held indefinitely and there is a powerful query tool available to interrogate it. More details can be found here.
mon, free network and website monitoring. There are many factors which affect Website availability and performance from end user perspective, namely ISP Internet connection, server location, server parameters, programming language, application architecture and implementation. One of the critical parameters is a selected Operational System (OS).
Sysmon is a tool to monitorise the state of one or more computers. Its based on a daemon and a php script. The first has to be runned in all boxes you need to check, the second calls the daemon and prints the state into a nice web frontend.
Spong is a simple systems and network monitoring package. It does not compete with Tivoli, OpenView, UniCenter, or any other commercial packages. It is not SNMP based, it communcates via simple TCP based messages. It is written in Perl. It can currently run on every major Unix and Unix-like operating systems.