I looked at 3 of the free bare metal hypervisors in the market:
I personally found Citrix Xenserver to be the best among the available baremetal hypervisors for the following reasons:
- A wide array of features required to run your servers available free of cost.
- Max support of 8 virtual CPUs vs. only 4 for ESXi
- Better hardware support for SATA harddrives out of the box [ESXi only supports SAS/SCSI - you need to hack it to make it work with SATA].
There are some hardware requirements for these hypervisors. You need to have 64 bit CPUs that support either the Intel-VT or AMD-V hardware virtualization technology. There is also a very limited subset of hardware that these hypervisors are certified with. Make sure to check these requirements before making the jump into bare metal virtualization. The alternative is to go with a hosted hypervisor [a.k.a Type 2 hypervisor] such as Sun's Virtualbox, VMWare Server, or Microsoft VirtualServer.