nagios is well-known as an alarming system.
munin offers the possibility to name warn and critical values. In single use munin will show items that are beyond their warn and critical values by link highliting.
Additionally munin offers the possibility to make use of nagios passive checks via nsca.
nsca is a part of nagios.
first one needs to configure the munin-master to make use of the send_nsca command.
second one has to configure the nagios master to also run the nsca daemon (either via inetd or as standalone daemon)
Since nsca documentation is very simple the guys from munin made a documentation on how to combine munin and nagios.
The advantage of this is that you get information upon changes immediately.