I often have a need to monitor some basic system metrics such as memory usage, disk space free, load average, and network usage. MRTG is an ideal solution because it is lightweight and can graph just about anything on a system. Many people only think of MRTG as a tool to graph network interfaces, but because it is just a way to use rrdtool it can be be used for practically anything.
Getting it installed is pretty easy as well. Most distributions provide a package for it, so it is as simple as running ‘yum install mrtg’ or ‘apt-get install mrtg’. Unfortunately the basic package is pretty raw and doesn’t by default know how to obtain much useful data. Over time I’ve compiled a /etc/mrtg/ directory that does a lot of the common things that I like to monitor. Simply uncompress this file to your /etc/mrtg directory and you should have some common useful metrics. You can further customize it per-server if you’d like to monitor some additional things.