Central Communications is a huge fan of open source tech.
When developing a cluster of virtual servers, we investigated all the options.
What we decided on was the open source option Proxmox.

Proxmox allows us to cluster real machines together.
These real machines host virtual machines, which we can move in real time between hosts.

Proxmox VE is a complete open source platform for enterprise virtualization. With the built-in web interface you can easily manage VMs and containers, software-defined storage and networking, high-availability clustering, and multiple out-of-the-box tools on a single solution