Solaris Containers (Zones) are exceptional and well proven as a powerful solution. xVM (Xen) has been rev'ed up to Xen 3.3.1 and is both mature and capable. VirtualBox continues to provide a solution that competes strongly against VMWare solutions, especially given that its got better cross-platform support, runs as fast as VMware Workstation or Fusion and lets not forget is free! And the powerhouse duo of ZFS and Project Crossbow's VNIC capability seal the deal creating a virtualization platform unlike any other.