Table of Contents
Linux examples
Note: All of these Examples are written for/on a Linux box. They can vary on your box, and you may have to adopt the command / evaluation expression.
If you want to run them e.g. on a Solaris server you have to do some research on your own 
But that's one of the main goals of this example collection: it should awoke your interest to create your own commands.
Network
I know that these values can be monitored by several SNMP driven plugins. But this approach is very simple and does not need anything to be configured specially. You can combine all network checks in one check_multi call.
Network packets incoming
check_generic -n "net_pkt_in" -e "IF=`awk '\$2 == "00000000" {print \$1}' /proc/net/route`; grep \$IF /proc/net/dev | awk -F: '{print \$2}' | awk '{print \$2}'" -w '>1000' -c '>2000' -y delta -p "pkt_in"
Network packets outgoing
check_generic -n "net_pkt_out" -e "IF=`awk '\$2 == "00000000" {print \$1}' /proc/net/route`; grep \$IF /proc/net/dev | awk -F: '{print \$2}' | awk '{print \$10}'" -w '>1000' -c '>2000' -y delta -p "pkt_out"
Network bytes incoming
check_generic -n "net_bytes_in" -e "IF=`awk '\$2 == "00000000" {print \$1}' /proc/net/route`; grep \$IF /proc/net/dev | awk -F: '{print \$2}' | awk '{print \$1}'" -w '>1000000' -c '>2000000' -y delta -p "bytes_in"
Network bytes outgoing
check_generic -n "net_bytes_out" -e "IF=`awk '\$2 == "00000000" {print \$1}' /proc/net/route`; grep \$IF /proc/net/dev| awk -F: '{print \$2}' | awk '{print \$9}'" -w '>1000000' -c '>2000000' -y delta -p "bytes_out"
Network errors incoming
check_generic -n "net_errs_in" -e "IF=`awk '\$2 == "00000000" {print \$1}' /proc/net/route`; grep \$IF /proc/net/dev | sed 's/^.*\$IF://' | awk '{print \$3}'" -w '>5' -c '>10' -y delta -p "errs_in"
Network errors outgoing
check_generic -n "net_errs_out" -e "IF=`awk '\$2 == "00000000" {print \$1}' /proc/net/route`; grep \$IF /proc/net/dev| sed 's/^.*\$IF://' | awk '{print \$11}'" -w '>5' -c '>10' -y delta -p "errs_out"
Parse ifconfig output directly
Another approach has kindly been provided by coffy from the German Nagios forum.
He directly parses the output of ifconfig to avoid permission problems on the /proc/net files. On the other hand this approach needs the knowledge of the particular interface to check.
Example: check network errors
check_generic -n Net_TX_Errors -e "ifconfig eth0 | grep 'TX packets' | cut -d: -f3 | awk '{print \$1}'" -w ">5" -c ">10"
Memory / Swap
Memory free
check_generic -n "proc_meminfo_memfree" -e "grep -i memfree /proc/meminfo | awk '{print \$2}'" -w '<5000' -c '<2000' -p "free_KB"
Memory dirty
check_generic -n "proc_meminfo_dirty" -e "grep -i dirty /proc/meminfo | awk '{print \$2}'" -w '>50000' -c '>100000' -p "dirty_KB"
Swap in use
check_generic -n "proc_meminfo_swapinuse" -e "grep -i swapcached /proc/meminfo | awk '{print \$2}'" -w '>50000' -c '>100000' -p "swap_KB"
Process environment
Interrupt number
check_generic -n "proc_stat_intr" -e "grep -i intr /proc/stat | awk '{print \$2}'" -w '>500' -c '>1000' -y delta -p "intr"
Context changes
check_generic -n "proc_stat_context" -e "grep -i ctxt /proc/stat | awk '{print \$2}'" -w '>1000' -c '>2000' -y delta -p "ctxt"
Blocked processes
Processes can be blocked eg waiting for a IO operation to be finished.
check_generic -n "proc_stat_blocked" -e "grep -i procs_blocked /proc/stat | awk '{print \$2}'" -w '>3' -c '>5' -y delta -p "procs"
Forks per second
check_generic -n forks -e "vmstat -f | awk '{print \$1}'" -c ">20" -w ">10" -y delta -p forks
Miscellaneous
RTC clock ok
check_generic -n "proc_driver_rtc" -e "grep -i batt_status /proc/driver/rtc | awk '{print \$3}'" -c '!~/okay/'
Smartctl disk /dev/hda
check_generic -n smartctl_hda -e "smartctl -H /dev/hda | grep 'test result' | awk '{print \$6}'" -c '!~/PASSED/'
ACPI remaining battery capacity (last full capacity in % of design capacity)
check_generic -n ACPI_battery_capacity -e "DC=`grep 'design capacity:' /proc/acpi/battery/BAT0/info | awk '{print \$3}'`; LC=`grep 'last full capacity:' /proc/acpi/battery/BAT0/info | awk '{print \$4}'`; echo \"$LC/($DC/100)\" | bc" -c "<50" -w "<70"
Nagios
Service checks number
check_generic -n nagios_services_number -e "/usr/local/nagios/bin/nagiostats -m -d NUMSERVICES" -w ">1800" -c ">2000" -p "NUMSERVICES"
Service checks per minute
check_generic -n nagios_services_checks -e "/usr/local/nagios/bin/nagiostats -m -d NUMACTSVCCHECKS1M" -w ">300" -c ">500" -p "NUMACTSVCCHECKS1M"
Service check latency
check_generic -n nagios_services_latency -e "/usr/local/nagios/bin/nagiostats -m -d AVGACTSVCLAT" -w ">30000" -c ">60000" -p "AVGACTSVCLAT"
Host checks number
check_generic -n nagios_hosts_number -e "/usr/local/nagios/bin/nagiostats -m -d NUMHOSTS" -w ">400" -c ">500" -p "NUMHOSTS"
Host checks per minute
check_generic -n nagios_hosts_checks -e "/usr/local/nagios/bin/nagiostats -m -d NUMHSTACTCHK1M" -w ">100" -c ">200" -p "NUMHSTACTCHK1M"
Host check latency
check_generic -n nagios_hosts_latency -e "/usr/local/nagios/bin/nagiostats -m -d AVGACTHSTLAT" -w ">30000" -c ">60000" -p "AVGACTHSTLAT"
