You ask? XenSource donates! I searched the net several times in the past, looking for an open source alternative to VMWare. Never found one, and now, it suddenly drops out of the sky ;).
However, it’s not the typical kind of virtualization most people know of. Xen is a mean ‘n lean layer, running the guest OS right on the physical machine! Using regular hardware, this requires modifications to the OS. But there is no need for special virtual device drivers. However, Xen 3.0 will support Intel’s VT processors, which means no guest modifications will be needed anymore. Because the guest OS runs right on the physical machine, Xen would achieve near native performance. Quite impressive IMHO.