There are a lot of great monitoring tools out there. I've posted many times before about Nagios and I could post still more on this great tool, but it's not the only tool I use. Another one is Cacti which is an excellent tool I've also mentioned before and it is mostly for graphing system resources.
Out of the box, Cacti will give you a lot of the basics especially when combined with SNMP. Disk usage, network interface usage, CPU, and memory. But what I really like about these great Open Source tools is that there are extensions readily available from the F/OSS community. With Cacti, you can extend by getting new host templates and data queries (and more). Here are some examples.
Disk IO - this is a new data query that tracks disk IO usage either in IOPS or MB/s. This is one of the simplest examples of how you can extend cacti. It comes as a xml file defining an SNMP query which you copy into your resrouces/snmp_query installation folder and as a data query template which you import through the Cacti UI. Once you've done this quick installation, you can add the disk io checks to any SNMP enabled host you are already tracking.
Dell PowerEdge Environment - this is another simple example which is the same as the Disk IO in that it is an SNMP query plus a data query template but there's 3 checks it adds. System ambient temperature, fan speeds, and system voltages. Its a great example of how Cacti as a generic tool can be tuned to target your specific operating environment whether you're a Dell shop, HP shop, or otherwise.
APC UPS Daemon - Another example of an application specific example. This one comes as a host template so its a collection of checks you can use to capture all the data queries on a host using APC UPS Daemon. A great example of where F/OSS tools *far* exceed the stock or closed-source tools provided by vendors. Rather than these cheesy brief inflexible views of how your system works as provided by APC that require overly large utilities to be installed, its quick, lightweight, and much more flexible to use the F/OSS tools.
Cacti is another of these great tools that works well in conjunction with other tools to give system administrators great insight into the operation of their network.
