If I remember correctly, I came across Nagios the first time in 2006. Since them monitoring systems and applications caught my interest and kind of stuck with me. My first public monitoring plugin was in 2008 (check_mysql_slavestatus), since then a lot of plugins have followed.
The following is a list of monitoring plugins I develop and/or maintain.
A plugin to check the validity of a Parallels Confixx License.
This plugin is EOL
more monitoring plugins
The following monitoring plugins are developed/maintained by other monitoring fellows but highly recommended:
check_openmanage Check the Dell server hardware by using this plugin in combination with OpenManage (installed on the server). check_openmanage can be used locally on the server (as NRPE check) or get the information from a remote server via SNMP. A must for Dell hardware! Written by Trond Hasle Amundsen.
check_hpasm A plugin to check the hardware of HP servers. Uses HP Insight Agents to query hardware status via SNMP. It's like check_openmanage but for HP servers. Written by Gerhard Lausser.
check_vmware_esx This plugin is a fork of the (once famous) check_vmware_api (before that known as check_esx) by OP5. It can be used for several checks on VMware ESX/ESXi servers (e.g. disk utilization, cpu and memory usage, etc.). It uses the VMware Perl API which needs to be installed on the monitoring server. You can follow by step by step guide how to install the VMware Perl SDK.
check_oracle_health This plugin, written by Gerhard Lausser, checks your Oracle database for fonctionality and usage (Connections, Flash Recovery, Process Usage, Session Usage, Tablespace Fragmentation, Tablespace). It can be executed locally on the Oracle server or also on the Nagios server by using a connect string.
check_logfiles Another plugin from Gerhard Lausser. This is a very powerful plugin using all kinds of regular expressions to search for patterns in log files. I mainly use this on Windows hosts for application log files.
check_nwc_health This plugin is one huge network monitoring plugin. It supports all kinds of different network devices, from Cisco over Checkpoint to Juniper and Brocade and so on. Instead of using many different plugins for each network device, this plugin tries to be the "all 4 one" plugin and it does the job very well.
more monitoring software
NSClient++ is a small client/agent to install on a Windows host. It will run as a service and receives commands from the monitoring server (using check_nt or check_nrpe). The ultimate monitoring agent to integrate Windows hosts into your open source monitoring solution. NSClient++ is maintained by Michael Medin.
Thruk is an alternative user interface which can be used for Nagios, Icinga 1.x, Icinga 2.x, and Naemon. It can use Livestatus or a MySQL database to talk to the monitoring system in the backend. Thruk features a very nice SLA reporting tool (as of this writing in March 2019 Icingaweb2 still is unable to create availability reports and the icinga2-classicui is not supported anymore). Thruk is maintained by Sven Nierlein.