Cacti is a network monitoring and performance graphing tool that works with SNMP to capture what is happening on the network and present that information in easy-to-understand graphs.
Cacti works by polling all SNMP-capable devices on your network and adding the information you've selected to graphs. In its simplest form, Cacti will give you the ability to create graphs for the most common aspects of host monitoring, such as disk space, loads, memory usage, NIC monitoring (bytes in/out of an interface), and so on. The browser based management console is nicely done, and plenty of support is offered on www.cacti.net.
Downloading Cacti as a virtual appliance avoids a somewhat complex configuration process that includes creating a virtual server, installing CentOS, and configuring Cacti. However, preconfigured virtual appliances usually cost a few dollars, as opposed to being free. An example of that would be the JumpBox version of Cacti, which runs $49 or $99 per month. A 7 day trial is available at no charge if you want to give that implementation a try, before going through the manual steps to create your own appliance.
As with any network monitoring tool, Cacti can be somewhat complex to set up, because of the dependencies on MySQL, PHP and other components. However, for those administrators who want a visual representation of what is happening on the network performance wise, it is pretty hard to beat the flashy, informative charts that Cacti can offer.