System Centre Virtual Machine Manager (SCVMM) has been my star of the show for our week of intensive technical training (TechReady). I love the way it produces Powershell scripts as you operate the GUI - taking the mystery out of how to script. Building a dynamic datacentre on any scale - even if just a handful of physical machines - is a realistic possibility now thanks to SCVMM.
SCVMM enables you to dynamically move running workloads from one physical machine to another based on the availability of "horse power". Switchover can take place in a second or two if you have shared discs - very cool indeed.