Graphs load average, disk usage, and network throughput.
Something simple I put together for some basic graphs, not a Cacti replacement. Requires RRD and presumably some web server to display the graphs.
Make sure you adjust the settings in the rrd_config file before you install.
Run the rrd_init_databases.sh to intialize the databases, then run ./update_rrds.sh &. You may want to put that in rc.local or some equivalent so it runs on boot.