VMware's vSphere Hypervisor is its starter kit for SMBs and others taking the virtualization plunge for the first time -- ostensibly to get them hooked on the platform as their needs grow. It's not open source, but it's free. "[It allows] plenty of flexibility and usability," Thorpe said. "We are running 20-plus virtual servers on a couple of ESXi physical hosts at the moment. Licensing fees only apply to my subset of Active Directory servers."