Much as I love to sing the praises of virtualization, I sometimes overlook that fact that virtual machines (or VMs) aren't exactly free. You need a license for each and every instance--and if those VMs are running Windows Server 2008 R2, license costs can add up in a hurry.

Or not, if you opt for Microsoft's little-known Datacenter Edition license. As discussed in 5 tips for managing Microsoft licensing costs, Datacenter has the potential to save your enterprise considerable cash. Here's an overview:

Windows Server is licensed per processor, and with today's multi-core processors the number of licenses required may far exceed the actual number of servers in a data center. But Microsoft's Datacenter Edition license offers "unlimited virtualization rights, meaning customers have the use rights to run an unlimited number of virtualized instances of Windows Server on processors licensed with Windows Server 2008 R2 Datacenter without purchasing additional licenses," according to Microsoft.

By way of example, the article notes that if you have a dual-core server running eight VMs, licensing costs work out about the same with or without Datacenter. But if you run, say, "40 virtual servers on a quad-core machine, you're going to save a lot of money by doing the Datacenter license," according to Microsoft licensing expert Cynthia Farren.

Okay, I'll fess up: this was news to me. Until now, I'd never heard of Datacenter Edition. You? In any case, it may help your large enterprise save a bundle.

While you're at it, keep reading to learn about Software Assurance, Windows 7 licensing strategies, virtual desktop options, and the value of negotiating. Good stuff.