The best open source alternative to OpenStack is QEMU, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to OpenStack and 19 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to OpenStack are Proxmox Virtual Environment (Freemium, Open Source), Xen (Free, Open Source), XCP-ng (Free, Open Source) and CloudStack (Free, Open Source).
Xen.org, home of the Xen hypervisor, the powerful open source industry standard for virtualization. It is a native (bare-metal) hypervisor providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently.
XCP-ng is a high-performance, enterprise-level hypervisor with a rich ecosystem and is the result of massive cooperation between individuals and companies. All commercial features are enabled with no commitment required to a support agreement.
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.
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CloudStack vs OpenStack opinions
Almost everyone thinks CloudStack is a great alternative to OpenStack.
retrack CloudStack has less moving parts in general than other Cloud Control Panels.
Danube Cloud is a cloud management software used for deploying, maintaining and automating cloud data centers and their processes. The project aims to simplify the whole server virtualization environment by automating all the repetitive tasks for the system administrator.
OpenVZ is container-based virtualization for Linux. OpenVZ creates multiple secure, isolated Linux containers (otherwise known as VEs or VPSs) on a single physical server enabling better server utilization and ensuring that applications do not conflict.