Check ESX is a plugin developed by op5 to primarily be used with op5 Monitor, but can also be used in Nagios installations, to monitor VMware ESX and vSphere servers. You can monitor either a single ESX(i)/vSphere server or a VMware VirtualCenter/vCenter Server and individual virtual machines. If you have a VMware cluster you should monitor the data center (VMware VirtualCenter/vCenter Server) and not the ESX/vSphere servers by them self.
Check ESX Plugin
Monitor VMware ESXi with your existing Nagios application