We've blogged in the past about Nagios, the open source monitoring software. Another great open source alternative is Cacti. Both Nagios and Cacti do a great job of graphing system resources. Like Nagios, Cacti is capable of monitoring your servers, as well as processor, memory, network, and disk utilization on your networking devices. After initial installation, adding hosts to be monitored can be completed using nothing but the web interface.
Installing Cacti is fairly straightforward. There are even customized distributions, such as CactiEZ, which is a CentOS derivative, to make for an even easier install. Cacti can even be installed on a Windows host. Cacti is incredibly useful right out of the box, but with a few plugins Cacti's functionality can really begin to shine. The following is a handful of plugins that I think no Cacti installation should be without. The CactiEZ distribution includes a lot of these plugins out of the box. All of these plugins can be downloaded at http://cactiusers.org/downloads/. CLOG - CLOG provides all of your Cacti logs in an easy-to-use interface built into the Cacti web GUI. This can be immensely helpful in troubleshooting issues within your Cacti installation. Discovery - This plugin can be used to track down SNMP-enabled hosts on your network that haven't been entered into Cacti yet. NTop - While this plugin only provides your ntop data within an iframe, it allows you a single interface to get a full overview of exactly what's going on in your network. Thold - "Thold" is short for threshold. This plugin allows you the ability to configure thresholds and alerting. If you want to get an email or text message when a disk is nearly full, or a network link is reaching max capacity, this plugin is what you need. Weathermap - This plugin is really cool. It allows you to draw maps of your network. Once the maps are drawn, you can assign graphs to the different links. These links will be updated in real time, changing colors based on utilization. This gives you a quick overview of how the network is performing. These are just a few of the many plugins out there that make Cacti such a great tool.