A great free tool (with a commercial counterpart) I experimented with today was Monitor from Veeam. It provides the ability to view multiple ESXi instances without purchasing virtual center. Now VMware’s virtual center does provide you alot more features and tools. But with Monitor you can view the resources of every ESXi server in your environment.
The following features make this a great tool to checkout:
Performance monitoring: Comprehensive data on resource consumption and workload, from vCenter, ESX/ESXi host, and complete down to the individual VM. Real time data consolidated in one place.
Hardware monitoring: Provides a complete picture of your datacenter, through CIM/SMASH API – track everything from temperature and fan speed, to ventilation, power and more – eliminating the need for service console based hardware monitoring agents.
Correlation of event and performance data: See all of your datacenter activities on a single page, in real time. No need to switch between different virtual machines, hosts or vCenters – all information in under one easy to modify view.
User interface optimized for monitoring: The interface was designed with monitoring, not control in mind.