One of the exciting developments in the VM world is VMware releasing ESXi for free.

ESXi is Vmware's 32 megabyte (yes megabyte) hypervisor that allows bare metal VMs to be run on a server. VMware server, which has been available for a few years as free, is different in that it runs as a process or service on a host system.

There are some pretty important differences that should be noted between the two systems. VMware was able to reduce ESXi to its tiny 32 megabytes because it has had all the management stuff that ESX has eliminated from it. This means that you can not SSH into it like VMware server or ESX 3.5. It also means a lot of ESXi's abilities only come available when you use VirtualCenter. ESXi also has a pretty limited number of servers it can run on. The big advantage over ESXi has over VMware Server is the ability to run many more VMs because of the lack of host OS. You also are able to connect it to iSCSI luns for shared storage and the VM "drives" are SCSI not IDE. One of the reasons why I would opt for ESXi over VMware server is for the simple fact that whenever you update the host operating system you may have to reboot the machine and taking a half dozen VMs off simultaneously. VMware server is more suited though if you are using a lot of appliances or want to be able to use VMware workstation.

For now until I can afford to buy a Foundation pack I will probably be running both types of machines. ESXi for any new machines I am bringing online and VMware server for the rest.