What are some alternatives?
When comparing Proxmox Virtual Environment and OpenNebula, you can also consider the following products
oVirt - oVirt is a virtualization management application.
OpenStack - OpenStack software controls large pools of compute, storage, and networking resources throughout a datacenter, managed through a dashboard or via the OpenStack API.
Virtualizor - Virtualizor is a powerful web based VPS Control Panel.
VMmanager - VMmanager is a virtualization management software, which presents a perfect tool for creating virtual machines, providing VPS services and building cloud infrastructure.
Apache CloudStack - CloudStack is an open source cloud computing software for creating, managing, and deploying infrastructure cloud services.
Scalr - Scalr is cloud management software for public & private infrastructure