OpenNebula is described as 'The Simplest Cloud Management Experience'. There are four alternatives to OpenNebula for Online / Web-based, Linux, Windows, Android and Android Tablet. The best alternative is Proxmox Virtual Environment, which is both free and Open Source. Other great apps like OpenNebula are OpenStack (Free, Open Source), CloudStack (Free, Open Source) and ZeroStack (Paid).
Apache's CloudStack is a Project backed by Citrix and designed to be a direct competitor to OpenStack. It is designed to deploy and manage large networks of virtual machines, as a highly available, highly scalable Infrastructure as a Service (IaaS) cloud computing platform.