RRDtool is the industry standard data logging and graphing application. Use it to write your custom monitoring shell scripts or create whole applications using its Perl, Python or PHP bindings.
RRD is the acronym for Round Robin Database. It is a system to store and display time-series data (i.e. network bandwidth, machine-room temperature, server load average).
Note: The download link provided redirects to the default download page for all platform distributions of RRDtool.