With the official announce of VMware vSphere 5 an historical era has ended: vSphere 5 will be based only on ESXi… ESX in no more available.
VMware ESX Server was probably the first bare metal hypervisor for x86 architecture and its first release was in 2001… Now, after 10 years, it has been retired. But this does not mean that there isn’t any support of existing ESX… support will be available still for several years, as reported in the official document:
It’s curious that, some months ago, another historical VMware products has reach his end of life: VMware Server (also noted as GSX Server)
So, actually, from the first products released by VMware only Workstation still exist.
PS: about the meaning of ESX and GSX see the VMware related acronyms or this recent post.