Take a look at ESXi vs. ESX: A comparison of features.
The table shows the differences between editions.
André
The vSphere products allow a choice of using the classic ESX install or ESXi. The free version of ESXi is identical in function with the licensed/paid versions. The missing piece is the ability to mange the free ESXi with vCenter or via scripts. The only options are to manage the free ESXi host(s) individually with the vSphere Client or VMware GO. There are packages that include the management tool vCenter and 3 licensed ESX or ESXi hosts. At the small end there is http://www.vmware.com/products/vsphere/small-business/buy.html