VMware ESXi is a hypervisor from brand with most experience in virtualization. It is free. But a more advanced version called VMware vSphere optimized for cloud computing. It has support for large number of OSes : linux , FreeBSD , Windowws nased OS like Xp , vista , 7 ,Me , 2000 , 95 , 98 , NT and msdos , Netwar , Asianux, IBM 0S/2, Mac OS and Solaris. It supports para virtualisation in case of linux as guest. Dell, Fujitsu-Siemens IBM and HP support is as embedded. It supports 8 way virtual SMP. It supports:
64 logical processing cores,
256 virtual CPUs, and
1TB RAM for host.
VMware ESXi can run some images in non vmware format using VMware vCenter Converter.
There are only two major competitors in cloud hypervisors Xenserver and Vmware. With VWware having much advantage of market.