ESXTop is a very powerful that that is used for analysis of host performance. It present live statistical data of storage, CPU, memory, network, and VM resource usage.
This article provides the last version of the "ESXTop Bible" pdf that was once found in the communities. While it is older it does still contain solid data concerning how to interpret the statistics that are found in in ESXTop.
ESXi
The attachment has been public face via VMware communities.
This is being place in this article since the posting has been broken in the communities articles.
The attachment is in a pdf format.