In the enterprise datacenter level it is not uncommon for sysadmins to use VMware's Distributed Resources Scheduler (DRS) coupled with Distributed Power Managem

In the enterprise datacenter level it is not uncommon for sysadmins to use VMware's Distributed Resources Scheduler (DRS) coupled with Distributed Power Management (DPM) to balance workloads and power down underutilized hosts. There are probably similar solutions from other vendors (disclosure: VMware employee)