VMware ESXi is an excellent product, but it isn’t cheap.  Yes, there’s a free version, but it has limited features and some significant licensing restrictions.  Need centralized management, live migration, etc?  Get ready to spend some serious cash.  The same is true for Xenserver and Hyper-V.

Enter Proxmox VE.  It utilizes KVM (Kernel-base Virtual Machine), which was developed by Red Hat and is included in the Linux kernel.  All nodes can be centrally managed from a web browser, and live migration is included.  It is a breeze to install.  Just download the iso file, burn a CD, boot and install on each node.  Proxmox runs on top of Debian Linux. One interesting twist is that Proxmox VE can run both fully virtualized KVM images or OpenVZ templates on the same host.  More importantly, it’s free!  However, commercial support is available when you need it.  Unlike some open source products, there’s not a crippled free version and a fully-featured premium version.  Furthermore, quite a bit of documentation and video tutorials are available.  I encourage you to give it a test drive and donate to Proxmox if you like it.

Further reading:
Proxmox: The Ultimate Hypervisor
Proxmox: The high-performance virtualization for the rest of us