Citrix has made XenServer 6.2 available to all users as a free open-source virtualization platform, beyond just the Xen hypervisor that has been apart of the Linux kernel and one of the leading Linux virtualization systems aside from Linux KVM.
Citrix has also launched XenServer.org, a paid version of XenServer that has maintenance/support, new CPU socket licensing, and simplified packaging/pricing. The announcement comes via this blog post.
XenServer was already mostly open-source but now Citrix has opened up the whole platform as the company believes "open source plays a strategic role in the future of virtualization and cloud technology and that only open source offers the opportunities for collaborative, open innovation and the economies of scale that these markets demand. By open sourcing XenServer, customers, partners and developers gain full public visibility into the ongoing development and future of XenServer and can directly engage with us to contribute new XenServer functionality, build deeper integrations and steer the architectural direction of the platform."