Sysstat
sysstat is a collection of command-line system and performance monitoring utilities for variants of the GNU Operating System such as Linux. This suite of programs can help you understand if a server being slow or unresponsive is due to memory constraints, high CPU load, high I/O load and so on and it's also got tools for pin-pointing what process(es) are causing what load. Learning the various tools in the sysstat package can be quite useful.
The Tools
There is, ironically, no actual sysstat program. The tools in the sysstat package (the package manager will install all of them when you install sysstat) are:
These tools are actually essential to understanding what is really going on behind the scenes on a server (and they are also useful for a heavily loaded personal desktop).
Links
sysstat's homepage is at sebastien.godard.pagesperso-orange.fr.