Monitorix is a free, open source, lightweight system monitoring tool designed to monitorize as many services as it can. At this time it monitors from the CPU load and temperatures to the users using the system. Network devices activity, network services demand and even the devices' interrupt activity are also monitored.
 

 Cacti is a complete network graphing solution designed to harness the power of RRDTool's data storage and graphing functionality. Cacti provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features out of the box. All of this is wrapped in an intuitive, easy to use interface that makes sense for LAN-sized installations up to complex networks with hundreds of devices.
 

 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 is the OpenSource industry standard, high performance data logging and graphing system for time series data. Use it to write your custom monitoring shell scripts or create whole applications using its Perl, Python, Ruby, TCL or PHP bindings.
 

 AWFFull - A Webalizer Fork, Full o' features!

AWFFull is a webserver log analysis tool. Mainly used to produce simple reports, it can also be used as the starting point for more detailed Web Analytics.

The output produced is a series of simple HTML pages and images,making it trivial to access the reports from practically any web browser. No additional configuration or set-up is necessary.
AWFFull does produce "at a glance" metrics to help identify problem areas within a site.