As a strike against the industry leading VMWare, Citrix releases it's Xen Enterprise Virtualization platform to the public for free. Note this does not mean it's been released OpenSource as the Xen hypervisor is, it's just free to download and use on as many installations as you like. The difference is that it's free as in beer, not as in freedom. If Citrix wanted to change the license back to pay only after it hooks a million customers it can.

BOSTON » 2/23/2009 » Citrix Systems, Inc. (Nasdaq: CTXS), the global leader in application delivery infrastructure, today unveiled a groundbreaking new version of Citrix® XenServer™ – the company’s enterprise-class, cloud-proven virtualization platform – that will be offered free of charge to any user for unlimited production deployment.  While basic hypervisors have been free for years, most have had limited practical use in real world environments.  With this new release, XenServer sets an entirely new standard for free virtualization with the addition of powerful new features like centralized multi-node management, multi-server resource sharing and full live motion.  Dramatically lowering the entry price of virtualization also helps address today’s challenging economic climate by making enterprise-class virtualization far more accessible to businesses of all sizes, regardless of budget.

Xen Enterprise is running in 5000 datacenters worldwide. Downloads will be available in March. Here's the official announcement.