VMware ESXi is server virtualization software written by VMware.
VMware ESXi was originally a reduced version of VMware ESX, that allowed for a smaller 32 MB disk footprint on the Host. With a simple configuration console for mostly network configuration and remote based VMware Infrastructure Client Interface, this allows for more resources to be dedicated to the Guest environments.
There are two variations of ESXi, VMware ESXi 3.5 Installable and VMware ESXi 3.5 Embedded Edition. It also has the ability to upgrade to VMware Infrastructure 3 or VMware vSphere 4.0 ESXi.
Originally named VMware ESX Server ESXi edition. Through several revisions finally becoming VMware ESXi 3. New editions then followed ESXi 3.5 and now ESXi 4.
Version release history:
VMware ESX 3 Server ESXi edition
VMware ESXi 3.5 Initial Release (Build 70348)
VMware ESXi 3.5 Update 1 (Build 82664)
VMware ESXi 3.5 Update 2 (Build 110271)
VMware ESXi 3.5 Update 3 (Build 123629)
VMware ESXi 3.5 Update 4 (Build 153875)
VMware ESXi 4.0 (Build 164009) (2009-5-21; 2 months ago)