I run...
VMware ESXi 5.1 on an old Proliant DL385 (2x Dual Core Opteron 2.4 GHz, 16Gb RAM, 6x 73 Gb U320 disks on SmartArray) and a QNAP TS459U NAS as storage.
I do not have a vCenter server (because that is what you pay for now) so I can only do manual off-line cloning and can't make snapshots.
I run 6 ~ 8 Linux servers on this and it works very well (and is FREE).
Microsoft is "free" for use when you have license for the appropriate Windows Server and there are restrictions on how many Windows machines you may run on it (2 Windows VMs on a standard W2K8 Server).
Biting the hand that feeds IT
