/usr/share/munin/plugins/mbmon_ is in munin-plugins-extra 2.0.19-3ubuntu0.3.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102  | #!/bin/sh
# -*- sh -*-
#
# Plugin to monitor Motherboard temparature using mbmon
#  (http://www.nt.phys.kyushu-u.ac.jp/shimizu/download/download.html)
# Copyright (c) 2005 Arne Schwabe
# All rights reserved.
#
# Redistribution and use in source and binary forms are freely
# permitted provided that the above copyright notice and this
# paragraph and the following disclaimer are duplicated in all
# such forms.
#
# This software is provided "AS IS" and without any express or
# implied warranties, including, without limitation, the implied
# warranties of merchantability and fitness for a particular
# purpose.
#
#%# family=contrib
#%# capabilities=autoconf suggest
what=`basename $0 | sed 's/^mbmon_//g'`
if [ "$1" = "suggest" ]; then
	printf "TEMP\nFAN\nVoltage\n"
	exit 0
fi
if [ "$1" = "autoconf" ]; then
	if $mbmon -c 1 > /dev/null 2>/dev/null ; then
		printf "yes\n"
		exit 0
	else
		if [ $?=127 ] ;then
			echo "no (executable not found)"
		else
			echo "no (mbmon could not read sensor values)"
		fi
		exit 0
	fi
	exit 0
fi
if [ "$1" = "config" ]; then
case $what in
	TEMP)
		echo 'graph_title Motherboard Temperature'
		echo 'graph_order TEMP0 TEMP2 TEMP1'
		echo 'graph_category sensors'
		echo 'graph_vlabel C'
		echo 'graph_scale no'
		echo 'TEMP0.label Temperature 1'
		echo 'TEMP1.label Temperature 2'
		echo 'TEMP2.label Temperature 3'
		exit 0
		;;
	Voltage)
		echo 'graph_title Motherboard Voltages'
		echo 'graph_category sensors'
		echo 'graph_order VC0 VC1 V33 V50P V12P V12N V50N'
		echo 'graph_vlabel V'
		echo 'graph_scale no'
		echo 'VC0.label VC0'
		echo 'VC1.label VC1'
		echo 'V33.label +3,3V'
		echo 'V50P.label +5V'
		echo 'V12P.label +12V'
		echo 'V12N.label -12V'
		echo 'V50N.label -5V'
		exit 0
		;;
	FAN)
		echo 'graph_title Motherboard Fans'
		echo 'graph_category sensors' 
		echo 'graph_order FAN0 FAN1 FAN2'
		echo 'graph_vlabel rpm'
		echo 'FAN0.label Fan 1'
		echo 'FAN1.label Fan 2'
		echo 'FAN2.label Fan 3'
		exit 0
		;;
esac
fi
case $what in
	TEMP)
		$mbmon -c 1 -r | sed -e "s/ *: */.value /"  |grep TEMP
		exit 0
		;;
	Voltage)
		$mbmon -c 1 -r | sed -e "s/ *: */.value /"  |grep V
		exit 0
		;;
	FAN)
		$mbmon -c 1 -r | sed -e "s/ *: */.value /"  |grep FAN
		exit 0
		;;
esac
 |