The best open source alternative to VMware Server is QEMU, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to VMware Server and four of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to VMware Server are Xen (Free, Open Source), Citrix Hypervisor (Freemium, Open Source) and UCS Virtual Machine Manager (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.