VMware just expanded product line introducing a hi-end tool for multiple ESX server (and multiple GSX Server in future) environments: VirtualCenter 1.0
VirtualCenter offers an advanced VMs control that GSX and ESX web management interfaces cannot offer. It also introduces VMotion technology to move without any downtime a virtual machine from one physical ESX to another one.
With VirtualCenter VMware eventually cuts off many third parties companies providing VMs managing products, like Leostream, Platespin, Dunes, etc.